I believe there's something in Apple's 48 page Terms and Conditions that says that that's against the law, but who reads that?
Apple is basically a walled garden unlike android. Jailbreaking your phone is breaking out of the walled garden enabling you to do more stuff with your phone like android. However, it makes your phone more vulnerable to certain viruses. If you jailbreak your phone, do it with a purpose.
Funnily enough, it also says in iTunes' Terms and Conditions that "You also agree that you will not use these products for any purposes prohibited by United States law, including, without limitation, the development, design, manufacture or production of nuclear, missiles, or chemical or biological weapons." @clairevbear if your iPhone's warranty is still valid, and your prone to dropping your phone, I'd strongly advise against it.
It's the principle of doing it for free that counts. EDIT: I didn't see the for free part at the end. Sorry, Star.
okay claire, here is some stuff you should know about jailbreak, i learnt the hard way. there are lots of troll tweaks that just destroy your phone, and what you will find is that you will get very frustrated with your phone after a few months with jailbreak. i jailbroke my iphone 4s at the start of the year and now it is SUPER slow and my contacts are starting to delete themselves and now i cant even send a text to anyone. and lots of your favorite games will recognize that you have cydia and automatically block you from going on multilayer and buying in-app purchases and that is really annoying. i could type an entire list of problems with jailbreak right here but i honestly could not be bothered, mail me on forums if you want to know any good tweaks and repos to install (like the ones that give you free apps and in-app purchases) but i would definitely not recommend jailbreaking a perfectly good phone.
I think you may be mistaking bugs with tweaks. Slowness generally would be due to the jailbreak itself and would be non-avoidable, and apps blocking you for using a jailbroken phone generally is a feature within the app itself.