The Ottomans made a difference, certainly. But it was a mistake for them to enter and one largely the result of Enver Pasha's drive to ally with one European alliance or another. Turkey had recovered somewhat from the declining years of the empire during the rule of the Young Turks but it was not prepared to play the kind of power politics role that Enver wanted.
That said though, the Ottomans made a difference. They successfully defended the Turkish core of their empire to the end of the war, from the Gallipoli landings and against the Russians in the Caucauses. They were eventually knocked out of the empire's Arab lands but Ataturk actually succeeded in holding the line in southern anatolia - roughly where the borders of the modern Republic of Turkey begins. Their biggest impact was against Russia, which their role in the war further weakened. The Allies' inability to take the straits made it impossible for the allies to resupply the Russians through the Black Sea.
You know, I've never understood why the Ottomans and Habsburgs couldn't crush the Balkan states during WW1. I mean, really, those two had dominated the peninsula for 100s of years and their only enemies were each other. You would have thought that the two front war between the two massive empires would have brutally squeezed the balkan states like a ripe zit.
A few reasons.
The Austro-Hungarian Army was not much of an effective fighting force. Many of its soldiers were slavs who had little interest in fighting Serbs and Russians - whom they felt solidarity towards. The Austrian army's invasions of Serbia were each effectively beaten back.
Also, not ever Balkan state was an allied belligerant in the war. Only Serbia and Montenegro were initially involved and Greece and Bulgaria blocked the overland route from Turkey. Greece did not enter until very late and Bulgaria joined on the side of the Central powers. Until it had done so though, Turkey had to cross through Bulgarian territory if it would have wanted to attack Serbia. Regardless though, Turkey had it's hands full before such an opportunity presented itself.