Q: Are there currently any reasons for me to jailbrake my iPad or iPhone?
A: Really at this point you should only jailbreak if there are apps you want that are available via Cydia and are not available via Apple. As far as unlocking you will only be able to unlock your iPhone 4 if the modem firmware is 1.59.00 (Settings–>General–>About).
Currently only Modem Firmware 1.59.00 can be software unlocked by ultrasn0w.
Getting a used iPhone 4 that is hardware unlock is very rare and the SIM unlocks (Gevey, Gevey Pro) are garbage in my opinion. It is possible that you can damage your phone inserting the Gevey, you have to redo the unlock every time you power down your phone, and I was getting pop-ups quite often telling me that my data was not working. You would be better off putting your $40 toward doing a Carrier Swap with some one (yes I do Carrier Swaps for $75).
Jailbreak is very easy now as you can jailbreak from the Safari browser on your iDevice by starting Safari and going to http://www.jailbreakme.com/. The people that are doing it for $20 are just ripping you off in my opinion when you can do it yourself in a couple of minutes with no skill at all.
I jailbroke one of my iPad last night just for fun and it took all of 2 minutes. Make sure you after you perform the jailbreak you install PDF Patcher 2 from Cydia which will fix the PDF vulnerability that allows you to run this Jailbreak from the http://www.jailbreakme.com/ website.
iPad1 and iPad2 come unlocked so you don’t need to get them unlocked. Watch out when you buy the iPad2 you don’t get the Verizon model which is CDMA and won’t work on a GSM network.
If you need a carrier unlock the best thing to do is sell and rebuy your iPhone for the right carrier or find some one to do a Carrier Swap with you. If you need a unlock because of travel buy a used iPhone with the baseband of 1.59.00 (Settings–>General–>About) or buy one new from apple as they are now selling unlocked iPhones for $649USD and $749USD in the US.