The bottle-neck for most computers is the hard disk IO. Compressing /usr can help speed things up. Google: squasfs usr for details. I wanted to compare xz(lzma 2) and gzip compression for in terms of bootup speed and firefox 5. Computer is ARCH linux Intel(R) Pentium(R) 4 CPU 2.80GHz, 2GRAM, single hard disk. KDM is set to auto-login to kde4. I rebooted a few times each to ensure fair numbers. I've removed the aufs option as it is not working for my current kernel.

All in seconds (less is better)
Normal uncompressed /usr (7.3G)
* kde4-splash; desktop; disk-io-stops; firefox
* 37;56;60;6
* 37;56;60;6

squashfs gzip /usr (3.7G)
* 25;45;55;4
* 25;45;56;5
* 25;47;57;4

squashfs xz (lzma2) /usr (3.4G)
* 35;75;90;7
* 35;76;91;7

Conclusions:
In terms of read speed: gzip is fastest then raw-disk and xz is slowest.

In terms of disk space: gzip and xz both compress the /usr partition to about half it's normal size. xz is smaller. But the compression still effectively requires more disk space because you need to keep your original /usr partition for upgrades plus space for your usr.squashfs file.

Effort -to- payoff ratio: given that you do NOT sit and watch the actual squashfs compression (takes a long time). Squashfs /usr saves you about 10 seconds every time you start your computer. Given you boot your computer and launch firefox once a day 6 days a week and sit and wait for it to boot. You will save yourself about 1 minute per week (4-5 minutes per month). So make sure you don't don't spend too long setting it up and testing and blogging about it like I have or else you will waste more time than you actually save in terms of speed increase.