Back when I had a jailbroken 3G running 3.something, it was noticeably slower. It ended up getting wiped by my 1 year old; I didn't re-jailbreak it and found it to be more performant.

IMO, unless you really need an app that lives only in Cydia, it's not worth the hassle. Tethering is cool, but seems like something I would personally need 1% of the time.