My own personal experiences: Canon lenses
EF 17-40mm F/4L Used this at a wedding back in April on a 5D, great for portraiture and group shooting and awesome value for money but it does tend to distort at the corners slightly at 17mm which can make people look taller (or wider) than they really are so be careful.
EF 50mm F/1.8 I own one, use it fairly frequently and it makes for a good studio portraiture lens as well as a general knock-about lens that you can throw around. Teaches you a lot about DOF and shooting in lower light. Downside is it isn't USM which means it's not as fast as one would think and it's very noisy when focusing.
EF-S 18-55mm Kit lens I think of this lens as a night out at the pub: It all starts out good but once you've had a few drinks you get cocky and it all goes pear-shaped. I still have mine and use it when I want to use my Cokin filter collection or my ND filters - I don't use it for much else. Recommend upgrading to an EF-S 17-85mm USM IS lens or EF 28-70mm F2.8L if you want better quality images.
EF 70-200mm F/2.8L It's big, heavy and it's white which means people will notice you in the street, at events or just in general when you use it and they will also automatically assume you're a pro. I've only used the non-IS version but even still the image quality and focal speed is superb. Throw a 2x converter behind it and you've got 400mm at the far end and F4.0 to play with. I used this for a full day at Wakefield Park without a tripod and believe me my wrists were paying for it the next day.
Zenitar 16mm F/2.8 Fisheye I bought it off eBay for $250 from a bloke in Belarus and being a manual lens it took a while to get used to, in the time i've owned it i've got some amazing photos out of it and it has more than paid for itself on a few occasions where i've used it. It doesn't work the best with APS-C crop DSLRS but there is still enough barrel distortion there for it to be noticed.