For people who want a good system (not an earthquake machine) with much improved sound and options at a reasonable price, you might be interested in what I installed. Installation is easy and the sound is quite good.
Subwoofer/amp - Subaru: $171.95 at Bruce Titus. Not an earthquake, but suprising good bass from a small box under the passenger seat.
Speaker Upgrade - Subaru: $123.24 at Bruce Titus. The car came with the optional tweeters. These sound better than most people give them credit for and are easy to install yourself. A huge improvement over the stock speakers.
Head unit - Jensen MP50110K: $142.50 on eBay for a new unit. Plays MP3s (how did I live without this?), clean powerful sound and CD changer controls. It had very good reviews for a Jensen product.
6 Disc CD changer - Jensen XCH 6 RF: $99.95 at Best Buy. Don't use the modulator and plug it into the head unit. Fits nicely under the drivers seat, which works out a lot better in a hatchback.
Wiring adapter: $6.00 at SoundDomain.com.
This is a lot of car stereo for $550. Any comments?

For $550 you could have done better. Should have kept the head unit and used an 5 channel amp and gotten after market speaker and a small sub.

Depends on what you want and don't want. What was important was was lots of music options and an MP3 player. Good sound was important but I did not want any sub boxes or disc changers cluttering up space in a hatchback. Everything fits under the seats and is out of the way. A sedan would be a different story, because amps and sub boxes can go in the trunk. It is a great convenience to have 7 discs to choose from and an MP3 player is a must. The stock head unit cannot do that and it does not have as clean a sound as the Jensen. (The stock single disc player is much better sounding in my opinion than the 6 disc that comes in the WRX). The Jensen head unit has plenty of power. Other manufacturers likely have similar good options, but the Jensens were a good value. The point is in the choices it provides. The Subaru upgrade speakers actually sound quite good and have no issues of clearance or need of finding spacers. The stock speakers were terrible, but the upgrades are able to hold their own. It is a no fuss, easy to install system (no drilling, no modifications). Yes, if a person just wants more sound or wants an earthquake machine, the $550 could be spent better, but not with the set of goals that I had.