Getting in here before nonsense gets posted, let me explain what jailbreaking does...
Jailbreaking gives you root(superuser) access to the filesystem. End of story. There is NO negative affect from that AT ALL.
Under normal circumstances(not jailbroken), apps are "sandboxed". Basically, an app cannot do anything to actually mess up the filesystem overall, because they are running inside this "sandbox". This also means that apps cannot interact with each other in any real way. That's why there aren't themes or widgets or anything like that in the app store. They MUST run inside of their own "sandbox" in order to be approved for the app store.
Enter Jailbreak. Now you have root access, which allows you to interact with the filesystem in any way you wish. Widgets, themes, speedups, apps that can interact with any part of the filesystem, etc. etc...
HOWEVER, and this is where jailbreaking gets a reputation for being bad for your device, you MUST be careful about what you install. A lot of users who don't know better go a little crazy and install tons of things that may or may not be coded very well. Badly coded cydia items can crash the device and require a restore, and everything you install that uses Mobile Substrate WILL have an affect on battery life, for obvious reasons. It's an extra service running in the background.
I have an iPhone 4 jailbroken on 6.1.3, and I have probably 6 or 7 tweaks altogether installed. It gives me all the extra functionality I want, the affect on battery life is minimal to non-existent, and I have actually made my device have virtually NO lag.
My personal list of tweaks:
BiteSMS- Gives Quick Compose and Quick Reply for texting
Switchy- Advanced multitasking bar
Gridlock- Lets me put the icons any where I want.
Zephyr- Gestures instead of using the home button.
IntelliscreenX- Basically supercharged notification center
TetherMe- Allows personal hotspot from iPhone
OpenSSH- Gives me SSH access to my iPhone
*I installed iCleaner long enough to use it to strip all the nonsense, then uninstalled it