Some of the offered software is more alpha than you would expect, especially the free apps.
Most sold apps in the Cydia Store are otherwise very stable but when you add Winterboard and a few themes you see the device rebooting twice or even 3x. You will also see MobileSubstrate catching most crashes but it is very bothersome to see it rebooted awhile you were wearing it and it received a SMS or email event and that caused a crash.
Pick and choose what you like and then when you stop playing with it, reset your iPhone and install everything in a specific order. Jailbreak, install your important apps and check for stability. Then Winterboard and your themes and check for stability again. If nothing bad happens, count your lucky stars and hope your next update does not make your idevice sick.
Adam