my friends said it was great
Oh, your friends told you. Misread the post.
----------
A few things:
Jailbreaking itself has no risk, but you may risk wearing out your battery if you install stuff that uses more processing power and therefore more current. Theoretically, software can destroy your speakers, but I doubt anything out there does that.
Jailbreaking has some great advantages, but it's more advantageous for some than it is for others. For anyone who is much more tech-smart than average (talking about the American average here), it has at least one perk. For many, it's a bad idea. I always jailbreak.
There is some instability brought by jailbreaking alone, despite what most people say. Rarely, the phone will crash and go into safe mode even without any tweaks installed.
This is totally unconfirmed, but I feel like jailbreaking by itself slows the device down a tiny bit. It also installs stuff like MySQL when you jailbreak, so it's not like jailbreaking
only installs Cydia.
Jailbreaking DOES NOT open your phone up to viruses, but it does allow you to use Cydia, which can give you Trojan Horses if you download from really sketchy repositories. You can also install OpenSSH from Cydia, which allows people to gain control of your device over the network if you do not change the root password! If you install OpenSSH, change your password! Cydia itself is harmless, but it lets you download anything. In addition, there is some honest software that is as bad as malware, slowing your phone down and causing instability because it's programmed badly.