Some very good information, thank you!
Over the past 20+ years I've had two 300mm f:2.8 non-IS lenses. In that time they've been the source of much if not most of my published, as well as personally favorite, work. In fact, I'm convinced that, during the interval I was without one, the quality of my photography suffered. Between the sharpness (on par with the best of L primes), its shallow depth of field and pleasing bokeh, is that good a lens that it is almost certain to make a better photographer out of you.
That said, there are reasons to be cautious about purchasing one. Almost all are 15 or more years old now, and some are as much as 25, making the availability of replacement parts an issue. I don't think FBW, by itself, is too much of a problem, but, rather, that Canon no longer supports this lens. You will need to find an independent source that's sufficiently skilled and resourceful to perform repairs, which, depending on where you live, will be anywhere from a moderate challenge to being totally out of the question. Case in point, a couple years ago, I needed to replace the rear mount. Pretty straightforward, right? Seeing that I couldn't get the part from Canon (and I'm a CPS member) so I could fix it myself, I took it to this area's best repair shop (I live less than an hour from Minneapolis-St. Paul). It turned out to take more than a couple weeks, as they had to order the part for a 300/2.8 IS, then, when it came in, machine it so it fit. Something to think about as you consider this lens.
What I hope to convey to you is that I highly recommend the 300mm f:2.8 non-IS. Few, if any, other lenses or cameras would give you as much "bang for the buck". But, be aware that, if it breaks, you may not be able to get it fixed. And, seeing how much you're almost certain to develop an attachment to this lens, THAT could be your greatest problem.
I do have a Canon authorised repair centre not too far from where I live, I know they supply all the Canon equipment to the biggest sports photography agency here in Dublin. I've heard that they've been able to fiddle around with equipment that is no longer covered by Canon but apparently it costs quite a significant amount, which wouldn't really surprise me.
It's hard to judge at the moment, it would be smarter to buy one of the newer IS versions to have it covered by Canon but the money isn't there to afford it. I know I'll eventually end up buying one and spending more money in the long run then I would have liked but I think, going with what can be afforded in the immediate future will benefit me by earning more money straight away.
I live with the reality that most of my gear could break down at any point, I'm still using 1D Mark IIN's (both beaten to bits), have a 300mm F/4 from 1995 (MF ring doesn't work) and a 70-200mm F/2.8 from 2001 (has more dents, bashes and scrapes than a car in a scrapyard), none of them have ever been serviced as I've never had the money at a set particular time. I seem to live on the edge with my equipment
When I'm able to fully get the money together I think I'll take the plunge and hope it pays off. Just have to wait for some more to pop up on the second hand market here in Ireland, or else head over to the US for a few days to pick one up!