This is an excellent lens for a bargain price. The AF is fast, the IQ is good, and it's not expensive. Sure, the AF isn't as fast as my 70-200 f4L, but honestly, it's pretty close. It has ring USM with FTM (full time manual) focusing - a feature usually not found in lenses of this price range. Sweet! The 100-300 USM has great build quality. The focusing ring is nice. The zoom ring is a bit on the loose side, but it's okay. The IQ isn't that great at 300mm, but that's to be expected of any lens in this price range.
Canon lists the 100-300 USM on their EF lens page, but I can't find it new anywhere. Used copies are generally easy to find. You can expect to pay $150-200 for a good used copy. It's a bargain. For about the same price as a 75-300, you can get a 100-300 USM. The 75-300 AF is SLOW. The 100-300 USM AF is FAST. The 75-300 doesn't have ring USM or FTM - the 100-300 does.
I think the focus is off a bit on my copy. This is the second one I've had; the first one was fine. I'll have to send this one in to Canon.
Just make sure you get the USM version - Canon makes a non-USM version, which is old and cheap and not worth it IMHO. They also make an L version, which can be found for around $300. It's discontinued and somewhat hard to find a copy. And the 100-300 L doesn't have USM.
This is a GREAT sports lens for people who don't have the budget for a 70-200. I think it should be recommended more often to people starting out who want a budget telephoto lens - this lens is up near the L-series lenses.