although the new ultrasn0w unlocks all basebands, I agree with propr'one, it's always safer to maintain your baseband. i'm still rockin version 4.26 on iOS 4.0.
for those who want a pwnage tool jailbreak but don't have a mac, you can use ih8snow:
basically a replica of pwnage tool for windows.
some of the main reasons i jailbreak:
SBSettings: most important app, gives you quick access to all your toggles by swiping the status bar, (you can turn on/of wifi, bluetooth, 3g, push, my3g, control brightness)
3G unrestrictor: to use apps that are restricted to wifi on 3G
install0us (even though i purchase all the good quality apps, at $.99-2.99 a pop its not a big deal
mywi: makes your iphone a wifi hotspot