So I was on a quest for more reach - my trusty 70-300 non-L wasnt enough for too many situations anymore. Realizing my budgetary contraints I did some research - and settled on a Tamron SP 200-500. Mounted on my T3i the equivalent 320-800mm is the reach I wanted. A bit sad to loose IS, and have found that using a tripod and remote release is pretty essential.
Unsurprisingly I've struggled a bit to get the shutter speeds I'd like without sacrificing ISO, but heres a few samples I shot recently of a heron and swan - included are 100% crops. Heron is shot at 1/640, F/6.3 (wide open) and ISO 200. Swan is 1/250, F/8 and ISO 400. Both are tripod shots with remote release.
Not as sharp as some more expensive "L" glass, but better than anything I had!