Has anyone used the 3rd party 70-200 lenses?
Are you happy with the quality, or do you wish you had saved up for longer and got the Canon?
Personally I have a 70-200 2.8 IS. Yes it cost a lot, and yes it is heavy, but the results from it speak for themselves.
But having said that, you need to pick a lens that is going to meet your needs (i.e. what sort of photos you plan on taking with it). I have a friend who uses a 70-200 F4 (non-IS) and is very happy with the results. Likewise I know someone who has a Sigma Bigma (50-500) who produces good results.
One resource you may find useful is Fred Miranda Reviews which agregates the results from people submitting reviews about the lenses they own.
I have a Tamron 70-200 2.8. It gets the job done and I've used it plenty for stage photography, but I always wish I forked over that little bit extra for the Canon. My particular lens always hunts and has problem with moving the autofocus motor.
I have the Sigma 70-200 (pre-IS), although Nikon mount, and I quite like it. Super fast focus, seems pretty sharp. Build quality is top-notch on mine.
I always go with first party lenses, only because of one silly issue:
The Canon's EOS lens system is proprietary. In order for 3rd party lenses to work, the manufacturers have to reverse engineer the communication protocols. There have been cases where older sigma lenses were not compatible with newer canon bodies. Sigma will happily "chip" older lenses to fix the issue (for free IIRC), but it's somewhat of a hassle.
I ran into a similar issue with my Sigma DG Super 500 flash being incompatible with my 5D2. Sigma will flash it, but I bought a Canon 580 EX II, and just use the Super 500 as a slave.
FWIW, I have read a few rumors that Canon secretly licenses their protocol to Tameron, but it was only a rumor.
I've used third party lenses when the quality was as good as or almost as good as the Canon lenses. One of the favorite lenses I've ever owned was the Tamron 17-50mm f/2.8 Di II. It served me well on APS-C bodies at less than 1/2 the price (at that time) of the comparable Canon EF-S 17-55mm f/2.8 lens. That lens and my old 50D were made for each other! The color was a tad less saturated and cooler than most Canon lenses, but since I shoot RAW pretty much all the time that was easily accommodated in post. And sometimes you want cooler and less saturated. The focus motor did make a unique (to put it mildly) sound, but it was fast and very accurate.
On the other hand, I thank myself for making the right decision and spending the extra greenbacks every time I shoot with my EF 70-200mm f/2.8L IS II USM. As of 2013, there is no other lens available, at least for a Canon mount, like it. From 70mm to 200mm. From f/2.8 to f/16 and beyond it approaches and sometimes equals the quality of comparable prime lenses. This is the first zoom I've ever used that doesn't have a soft spot at one end or the other (or in the middle), or a CA problem at a particular aperture and focal length. It just works as it should!
When Canon introduced the EF 70-200mm f/2.8L IS II USM a few years ago it was proclaimed by many to be the best zoom available. It was the first of several updates Canon has rolled out using supercomputer aided design and evaluation to produce lenses the best designers in the world only dreamed of creating less than a decade ago. Since that time they have done the same thing with many of their "Super Telephoto" collection. The new EF 24-70mm f/2.8L II performs even better at its designated focal lengths than the EF 70-200 f/2.8L IS II does. The Tamron 24-70mm f/2.8 Di VC that was introduced about a year ago is a little better optically than the original EF 24-70mm f/2.8L, but it doesn't compare to the II. For one half the price it comes close enough for a lot of folks.
In the end, everyone has to decide for themselves what the relationship is between "close enough" and "good enough", and how much the difference, if there is any, is worth.