I've had pioneer DEH-P80 (380), DEH-P7500, DEH-P1400. I am currently using a Kenwood Excelon KDC-X789, Which is by far the best I've used ($I paid 279 from a local shop owner but going price is around $399) It all depends on what youre going to do with it...If youre going to be putting amps up to it, you want something with a high output voltage (4V+). If youre not, you want to go with something that has a high rms (22+: Most pioneers have 22rms, but the P80 has 27rms) Also, is satellite radio something you want? Do you want somehting that has a sound stage equalizer for the front speakers? It has alot of factors. (I love this, i have been doing installs for over 4 years) PM me or post what you really want to do it and ill let you know what I'd most recommend. The clarions are also very nice, but theyre a bit pricier. This is a good time to get a HU, Because alpine, pioneer, kenwood, and sony have all just com out with their 2006(7) line of products, so you can pick up most of the past year's HU, with the same functions, for a much lower price.
Best of luck, and like i said let me know what yorue looking to do...