From what I've seen over the past few years, jailbreakers tend to fall into one (or more) of three camps:
1) Hackers/Homebrewers. They typically install feature modifications such as MapsOpener, 3GUnlocker, SBSettings, custom Bluetooth stacks, shells, file browsers, Wifi hotspots, emulators, etc.
2) Themers. I usually see people that fall into this group installing sometimes extensive GUI hacks. Some of them are beautiful and intensely elaborate, some of them are very simple and effective (five icon rows? who wouldn't want that? Also, home screen folders before apple put them in were amazing. I feel that f.lux also falls into this category), and some of them seem to exist only to take up battery and cpu cycles.
3) Pirates. Get ALL the apps!
I myself fall into more of the first group. I experimented with some themes back when I had my first iPhone but I found that most of the themes tended to slow the system down (not sure if this is still the case; we're talking about the older versions of iOS here). I even had a theming app brick my phone once. My mods are pretty conservative; between 10 and 15 Cydia apps, almost all of them functionality based, with the occasional experimental app here and there. Once it's setup, I'm happy. I've never had any issues with my phone slowing down or bricking. I seem to have this much in common with many of the jailbreakers falling into the first group; there are also some guys i've seen who take this to the extreme.
Just an observation here...I've noticed that those in the second group tend to get 'bored' with their phones quite quickly. They also seem to result in a lot of the 'why is my phone slow' and 'uninstall jailbreak' threads around here.
The third group seems (to me at least) to be the most impatient about a jailbreak coming out, utilizing the tethered hacks, then complaining when it all doesn't work amazingly.
Jailbreaking isn't for everybody, and I'm not meaning to say that I'm 100% right or flame anyone who does/doesn't jailbreak. Just thought I'd write down what I've noticed over the past few years.