Thanks for ALL the valuable feedback.
newworld666 wrote in post #9656762
I am not pro and I have a 300L2.8 non IS .. and it's surely de best sharpness I can get between my for lenses 24L1.4II, 85L1.2II and 135L2.0 ...
I use it for sport (F1 QuadBike) and also Dog shows. But, I start to use it as walkaround for candids or landscape since I discovered Black Rapid's R-STRAP. With a 5DMKII it 's around 4kg so quite easy to carry without being tired.
http://www.youtube.com/watch?v=soMGTUWAsj0
Just to makesure I understand, you believe this lens is sharper than the 85L II? I'm a bit intrigued that people use the 300 as a walk-around lens though.
Any reason you didn't get the 400 f/2.8 IS? Also, have you ever wished you had more reach and had purchased the 400 instead of the 300?
neil_r wrote in post #9657140
The Non IS is very sharp if you use it right so I guess the IS version is too. I also have a 500 f/4 which I use for motorsport and as with quite a few people I know I turn off the IS.
Can you please help me understand why many people turn off the IS? I thought the lens auto-detected tripods.
Michael15 wrote in post #9657302
I also have the EF 300 f2.8 and believe me I'm no pro!! I was going back and forth in my head about this lens....1. should I just rent....2. is it to much $$ for a hobby...3. will i look like an idiot with it at events???
I'm not a pro so I've been a bit concerned I'll look like an idiot with a big lens too. BTW, how do you get the lens in and out of events? In the case? I'm not a big fan of used equipment because I'm a bit too risk-adverse in that area. 
Neilyb wrote in post #9657609
I use it alot over the winter when I am out shooting from the hide, summer it is more for travel and used less. But for those shots I do take it is more than worth the money!
You take the 300mm with you when you travel?!
rshmn wrote in post #9657725
I've owned the 300 2.8 IS since Friday. Yes, it's probably overkill for what I do, but I love it! I'm shooting a local football team with it starting in the spring, and will get an extender for wildlife during the summer and see how I like it!
Any reason why you didn't get the 400?
bobbyz wrote in post #9657912
300mm f2.8 IS here as I have 500mm f4 IS also. Only hobbyist. Use for sports mainly. Only thing better than 300 as far as outdoor sports are concerned is 400mm f2.8 IS but it is heavier and more money.
Sharpness wise very very small difference which won't be noticeable in prints. If you have FF, then I would go for 400mm f2.8 IS as 300mm can be limiting depending on the sport.
I don't think my wife would love me getting both 300 and 400 at the same time, so it's going to be one or the other (at least for now). I've got FF bodies now, but I'm flirting with the idea of getting a Mk IV. I figure if I get a Mk IV and a 300mm, it's about $9K. A 400mm is about $7K all by itself.
neurorog wrote in post #9658042
Forgive me if I am giving information that you don't need

, but there are less expensive ways to get greater reach than your 200mm. Maybe consider 300 4, 400 5.6, or a 100-400 or one of the sigma zooms. If you find that you enjoy taking pictures at those lengths, but you require either more sharpness or better low light capabilities, you can always sell one of those and purchase a 300 2.8 or 400 2.8.
Information is always appreciated. I didn't consider the less expensive options because that always seems to result in regret for me. Also, I loathe buying and selling stuff and every time I have thought I would sell something, I end up keeping it somewhere or giving it away.