My understanding is that the Germans merely circumvented it as they invaded Belgium, the Netherlands, and France. However, as the war went on, did the line remain "intact?" If so, did the line provide any advantages or difficulties for the French or Germans, respectively?
Apologies if this question has been asked before (I would love a link to that place, if so).
I actually respectfully disagree with /u/Warband14.
For what the French planned and wanted, the Maginot line was a cost-effective way to achieve that.
The whole Maginot line costed about 5 billion francs 1930-1939, or about 2% of the French military budget for those years. The French would not have gotten more tanks or better reservist training out of those money.
The Maginot Line was intended to funnel the Germans through Belgium, which it did and did quite well. In ww1, the Germans had attacked not only through Belgium, but also reinforced their left flank in Elsass-Lothringen/Alsace-Lorraine in the hope of achieving a breakthrough there too and a double envelopment after the failed French attacks of Plan XVII.
The French knew their population was only about half that of Germany, and planned for a long war in which the British would build up their army and join them and strangle the Germans out of food and vital raw materials through the blockade of the Royal Navy. They were right that the war was a long one and that the Germans did suffer from the lack of oil, rubber and other vital raw materials, but the war was just not long for them.
The Maginot Line was intended as a force multiplier and a deterrent to German attacks directly on France, allowing the French to free up their best and above all most mobile and armoured forces to fight the Germans in a decisive battle in Belgium. In this role it worked very well. It was also a cost-effective way to achieve this, considering how little of the total military budget was spent.
The Germans never did attack the Maginot Line head on and the French never had to worry about that sector of the front, until the front had reached territory behind the Maginot Line. Even then, the forts held out pretty well and the Germans left them alone.
Not very effective at all, and I would argue that it ultimately hurt the French war effort. The Maginot line was built in such a way that it trapped its garrison within and the troops locked inside the Maginot line would not be able to attack only defend. The Maginot line did prevent a frontal attack by Army Group C but, as you already mentioned it was a useless endeavor, as Germany's main line of attack was through the Ardennes. (they managed to capture some smaller forts but no major ones).
After being captured the Maginot line was of little use. It wasn't used by the Germans to defend against allied invasion, since the Germans had their own defensive line called the Siegfried line. There was some minor fighting around the area of the line in the closing days of 1944 and early 1945 as part of Operation Nordwind, but nothing major.
The Maginot Line represented France's defensive mentality during the war, it was an old school way of thinking more rooted in World War One than in modern combat.
Source:
Second World War John Keegan.