Jailbreaking almost by definition makes your iDevice more unstable than it was out of the factory, just because all iDevices are locked down out of factory, so you can't make changes to the system files. Because system files are not user modifiable, that by definition makes unjailbroken devices more stable than jailbroken ones.
However, some jailbreak tweaks are really very stable, the ones that are made by established devs with lots of experience. When I jailbreak, I just install these trusted tweaks, and then don't tinker with the device for months at a time. Once I've picked and installed my tweaks, my devices run so stably I forget they are even jailbroken. However, it took some trial and error before I figured out which tweaks were stable, and I usually end up crashing and having to restore at least one device after each major jailbreak release before finding a stable configuration.
So I think jailbreaking is exactly like modding cars -- it can be lots of fun, but you end up spending lots of time on it to get things just right, and things tend to go wrong before you get there. Personally, for me, the end results are worth it, but your mileage may vary. You need to be willing to restore and start from a scratch if/when something goes wrong.
