I have an Alpine CDA-7875 Head Unit. Was going to get a 7873, but was sold out, and got a nice deal on the 7875. I must say that over the years, I've experience Alpine head units more out of coincidence than choice.
Have had two Volvos, and their OEM head units were Alpine. Sufficient for stock, but definitely not spectacular. Just did a nice system in my 850, and must say that the Alpine head is very good. Just to summarize what this head unit provides:
Bass Control - level, center frequency, and bandwidth of bass
Treble Control - level, center frequency
Subwoofer Control - full control of subwoofer (very useful feature!)
Bass Forward - allows more bass to go up front, but after a while, it's best off
High pass/Low pass filters
Two 4V pre-outs
Rated at 60 x 4 (definitely a strong head unit)
That's just the sonic goodies....it has a nice interface, graphics, remote, etc. Controls changers, is XM Radio ready (but u'll still need to buy the receiver though...at around $300 =), and has the cachet of alpine.
If you look at it from a broad perspective you'll notice that Alpine SPECIALIZES in head units. They make base line models with tape players, and go all the way to dvd systems, navigation, in dash changers, and all in one units. Some of these units top out around $3000, so you know they're not playing.
Granted alpine also makes speakers, subs, and amps, but I feel that they clearly take the cake in HUs. i've got a 10" bandpass sub that's nice and affordable, and sounds nice, but I know there are better ones out there.
So, if you can swing in the $400, i'll promise you quality!
Take Care,
Fellippe