Just scooped one up off eBay for $100. Can't beat that for a decent zoom - wondering if its really "l series quality"? I've heard good things regardless, but I like the opinions I get here best
If you do not have a telephoto zoom lens now, for $100 it is not bad at all. Although I think the EF-S 55-250mm maybe slightly better at the long end (based on reviews) but it cost a little more.
I had the 70-210mm f/4 back in 2008 when I got my first DSLR (also in 2008). And this is one of the photo I took with it.
1/400, f/5.6 at 210mm with ISO 400 on EOS XTi body.
This lens use push/pull method for the zoom instead of the 2 rings setup. Some people do not like push/pull type zoom but I am not against it. In fact zooming and manual focusing can happen at the same time.