I haven't jailbroken since I had a 3G.
Has it become more stable? How is the Cydia store? I've just never seen anyone with it recently either
Stability depends on what you do with it. I find a plain JB to be quite stable. But, adding crazy tweaks can change that. I didn't like the one I tried that allows folders to hold more than 12 apps, for instance. I have stayed away from Winterboard and Activator, too. Too many options kills stability, I just happen to disagree with Apple about where to draw that line.
Sometimes you can fix it. I have some tweaks for the lock screen, and it would crash Springboard (basically killing everything) if a system alert came up at the same time, mainly the battery warning. So, I found a tweak that altered the battery warning to prevent the crash.
My main reason for it is SBSettings, which I put at the top of Notification Center, for easy access to toggles like vibrate, wifi, brightness, etc.
On my i4, speed became an issue, or on my iPod 1stGen before that. The more tweaks you have, the more iOS slows down. Esp all-encompassing stuff like Winterboard, which I stopped using. I think the i5 will be great with a JB.
Cydia is ok. People put way too many ads in their info pages that can take a while to load, and there are no image placeholders, so it just changes the screen suddenly if another ad loads above where you are looking (like a webpage might do). But it works fine, you just have to be a little patient.