In the spirit of comradeship, despite this being non-RFF, I can offer my experience as an EOS user with Canon and third-party lenses. I have the Sigma EX 70-200/2.8 and have trialed it personally vs the Canon 70-200/2.8 L IS. The Sigma performs very near the Canon and is great value for the money. I also have the Tamron 28-75/2.8 - a very fine lens and not far off the Canon 24-70/2.8 L at less than 1/3 the cost. I have other Canon primes, but have not been able to compare them to third-party primes, so I'll refrain from comment there.
There is little doubt that lenses such as Canon's 200/1.8 and 85/1.2 are peerless, but you pay a great deal for such equipment: $1,500 for the 85/1.2 and $3,500 for the 200/1.8 used. If you've got the bucks, go for it. You won't be disappointed. However, if funds are less available, the 3rd party option is a great one. Shop carefully, though. All the lens makers have relative dogs in their product lines, third-party or original manufacturer.
Btw, compatibility has never been a problem for me. Although I still remain a film guy, I have shot my 3rd party lenses using other folks' digital bodies (10D, 20D, 1D) without any issues. Older, used lens may prove problematic and might require special service, like re-chipping.
Good luck!