I've got an Sony XAV-601BT.
Most important feature to look for in a head unit is usability/user interface. You're gonna be using the thing everyday.
It's got some chunky icons easy to tap while driving (or iphone like grid styled ones)
The Sony's decent for my needs:
got Bluetooth
AUX,
2x usb ports,
iPod (by the USB cable) support
A/v input (if you want to install a rear cam/ps2 etc),
plays DVDs too as well as mp3s
MP4 video files off the USB stick also.
If you got an android/galaxy s3 - you can mirror your phones screen and touch it through the HU, too unusable while driving tbh.
No Satnav, unless you fork out for the TomTom kit.
Sound output is quality, all depends on the source/bitrate of the track though.
Best part about the HU I found was 'Rear Bass Enhancer', basically turns your back speakers into subwoofers by only passing low frequency sounds.
I paid around £210 off ebay, suspect they'll be going for less now.
Literally did the install myself. Had to by a few leads off ebay too, and just followed tutorials on this forum. You wont find it too difficult
Pickles: