There's already numerous good answers- see /u/scarlet_sage 's links- but to give it a bite sized answer, the reputation is unwarranted and largely a product of the History Channel, and a guy- Belton Cooper- who's job in the war was recovering and repairing these vehicles and would later go on to write a a book- Death Traps- about his experience.

And were it just a matter of dealing with the book I would simply write, "Cooper spends real page space to feeling out a conspiracy theory that the M4 got the nickname, "Sherman" to snub noses at the south, even though the practice of naming American tanks after Civil War generals was something British Commonwealth did to differentiate between domestic products and American ones." and leave it at that. Or I could bring up the fact that he blames General Patton for delaying the Pershing from deployment in Europe until the war was virtually over, while neglecting to realize that Patton had nothing to do with the tank's development, testing, or production. Never mind the optical bias that having a job where you recover shot out tanks all day would give you. Belton Cooper doesn't do academically honest things- like looking up actual statistics on the performance of these tanks- and it shows.

As to the actual tank itself, first, there is actually no solid evidence that the tank was ever referred to as a 'Ronson.' The Ronson lighter company was an American brand- not British- and they specialized in higher end products. Your grandfather might have had a Ronson but you as an 18 year old who signed up in the dog days of the Depression? No, you did not. You had a Zippo. Similarly it wouldn't exactly make much sense for the Germans to refer to an American tank with slang meant for the British. There is no first hand sources that actually account for these nicknames- and it's not like that'd even be weird, we can trace the emergence of Kilroy- and absolutely no documentation at any level of military, government, or journalists that reflects the usage of either term. And they were perfectly willing to be critical of the tank's design.

Which feeds into the second point- yes, the M4 Sherman was prone to bursting into flames. As was every other tank in that era. It had nothing to do with the choice of engine and fuel types- Germans were using the same gasoline every but the Soviets ran their tanks on- but instead how munitions were stored in tanks. Back in those days you basically stuck rounds in one of three places- stowage bays underneath the seats of the crew, in ready racks to be rapidly drawn from and fed into guns, and everywhere else munitions would fit. Turns out, having the enemy's rounds strike your own rounds is a really bad thing. Best case scenario you got a mess inside, worst case scenario it sets off a chain reaction which ends with the entire tank going off like a firecracker, potentially with enough force to dislodge the tank's turret.

The important thing is that where comparable tanks- the Panzer 4 was less protected than the M4 Sherman, and had the same design flaws- didn't really change the interior cabin geography at any point in the war, the Americans made real effort to fix the problem. Crews were- although by all evidence this didn't always happen- requested / ordered to not stuff their tanks full of ammo anywhere it'd fit, where there used to be ready racks on virtually any flat, vertical surface (meaning that rounds that would land near the center of gravity on the tank that penetrated were almost guaranteed to hit at least one of these racks) they reduced the count and stored spare rounds under the crew, and eventually the US developed wet stowage racks which significantly decreased the chances that a rack, when struck, would brew up. By the end of the war the M4 Sherman actually had one of the best- if not the best- survival rates of any tank design.

Was the Sherman used incorrectly in war?

This stems from a few misreadings and misunderstandings. If the M4 Sherman was ever intended to be an infantry support tank (meaning that it was less intended to fight armor and more intended to disrupt and destroy dug in positions to support advancing infantry) it was a designation that ended very early in the tank's development. Furthermore, there is absolutely no binding documents that mandate the tank be used as such. While Tank Destroyer Doctrine stipulated that the US army was supposed to have these organic units of dedicated tank hunting assets (tank destroyers, AT guns, ect) that would move from hot spot to hot spot defeating armored pushes, this is not something the armed forces really ended up using. Americans in Europe faced an organized German offensive- the kind of thing Tank Destroyer Doctrine was tooled for- exactly once in the course of the entire theater (The battle of the Bulge) and the rest of the time were on the offensive.

The M4 Sherman was perfectly adequate at fighting armor and wasn't used 'incorrectly.' Although reading into the history of the 75mm gun- the M3 75mm gun was lifted in near whole cloth from a WW1 era French field gun, which may lead to some of the misunderstandings surrounding the tank's role- might give you a misunderstanding as to it's intended use, AP rounds were more than adequate at fighting tanks in it's class, and there were certainly more sophisticated rounds available (IIRC it was a composite capped?) that were simply never issued. And when it became apparent that they needed a gun with more anti-tank punch, the 76mm was perfectly adequate. With specialized HVAP rounds it could perform as well as the best German AT platforms in or above the M4 Sherman's class, and even then the need was thought to be so limited that crews seldom had very many of these rounds.

It is rather telling that while an up-armored model of the M4 Sherman existed- the M4A3E2, or 'Jumbo' Sherman- in a wider Sherman production numbering nearly 50,000 strong, only about 200 of these up-armored models were made, and they had protection on par with German Tigers.

Or was it the fault of inexperienced or bad crews who didn't know how to use it effectively?

While there is definitely something to be said for the naivety of American soldiers at the onset of the war, and that at times their training left something to be desired, there is no evidence to suggest that American crews contended with the same conditions their Soviet counterparts had to deal with which often lead to tanks being abandoned due to the crews being ill trained on maintenance, maneuvers, and performance of their tanks. Early in the war American infantry may have been using tin cans full of flour as a stand in for training with hand grenades, but apparently tank crews understood well enough how to operate and maintain their vehicles.

More can be written on this ... well, that's the standard wording that people use, but there are currently 8 articles in the FAQ section "Sherman Tank and American Armor" .

The second one is "Was the M4 Sherman a good or bad tank?" by /u/TheHIV123 . It is a relatively short read, and there are child replies with more info.

The first one is on Common myths surrounding the M4 Sherman tank by /u/the_howling_cow . It busts a lot of misstatements in its 9 parts of text.

Other links in the section cover specific aspects.

This is not to discourage discussion. Further questions, data, and debate are always welcome.

I saw another user in a different thread, u/The_Chieftain_WG, who may be qualified for this. Tagged for visibility. Do you have any thoughts on this?