The effect of tanks is a difficult thing to quantify, but for various reasons, tanks were considerably less effective in WWI than in WWII.
The problems that beset these early machines included very poor:
In addition, tanks did not make an appearance until September of 1916, at the Battle of Flers-Courcelette where of the 49 tanks available, only a half actually managed to go "over the top" and into battle. Even fewer actually made it to the German trenches.
They were incredibly fragile machines. Not only were they highly prone to mechanical failure, they struggled to cross boggy ground and would often get stuck in craters.
For example, in the battle of Messines Road Ridge, the tanks couldn't get forward at all due to the heavy rainfall the night before. Note that this didn't stop the attack being a huge success - sophisticated infantry/artillery co-operation and careful planning being sufficient for a 'bite and hold' break-in operation.
At Neuville St. Vaast, an officer recorded
We reached hunland, the shelled area which after the night's rain had turned into a sponge, quite unable to bear 30 tons or so. Before the Boche second line was reached, every commander was outside, picking a way for his tank, but even that method was no use and halfway to the Lens road, every single had bellied and remained so despite two to three hours spade work by the crews
Being very large and slow, they were easy targets for artillery and the armour was thin enough to be punctured by over-powered rifle rounds (at least until the Mk IV appeared in mind 1917). The History of the 123 (German) Grenadier Regiment states
The lt of the 3rd MG company... waited for a British tank to come within 150 meters of his gun and then fired 1,200 rounds of armour piercing ammunition against the tank's side. The tank was disabled within 50 meters of our front line. The tank was set on fire from at least 3 shots striking the engine and the resulting fire set off the ammunition. Seventy seven rounds were later counted which went through both sides of the machine
What's more, they were absolutely dreadful to fight in. There was no separate engine compartment and no provision for fume extraction.
Temperatures could exceed 40 degrees, carbon monoxide, petrol and cordite fumes would choke the crew, the noise was so extreme that the driver had to signal gear changes by hammering on the hull with a spanner.
Crude armour plate was prone to spalling from small arms fire, so little flakes or paint and steel would fly off the hull. The crew had to wear a special helmet with goggles and chain mail face covering because if one of those flakes got in your eye or if you breathed it in, you would be in very severe discomfort indeed.
Conditions in the tanks were so bad, crews often needed several days to recover themselves from a single days' action.
They were also hideously prone to fire. The British used specialist recovery crews to extract the charred corpses of the crew. They were forbidden to talk about their work to anyone else.
The availability rates for the battle of Amiens, 1918, tells its own story.
•August 8th, 1918: 414 available
•August 9th, 1918: 145 available
•August 10th, 1918: 85 available
•August 11th, 1918: 38 available
•August 12th, 1918: 6 available
That's an attrition rate of up to 80% to 85% per diem. Thus tanks could be a useful adjunct to a set-piece operation, but they by no means revolutionised warfare as they did in the second war.
Sources
Command and control on the western front - Todman & Sheffield
White Heat - Terraine
Forgotten Victory - Sheffield
Band of Brigands - Campbell