I have the iPhone 3g. I've had it for well over a year now and I still love it.
If you're thinking about jailbreaking your phone, there are a few things you need to consider:
1. iTunes doesn't like jailbroken iPhones. You'll need to use alternative software to update your music, videos, playlists, etc.
2. Apple says it is "illegal". NEVER show off your jailbroken phone to anyone in an apple store or in an AT&T(or whatever carrier you're using) store.
3. Jailbreaking your phone might cause it to be a little laggy. Personally, I have never had this problem. But, apparently others have.
4. Theme customization. You will NEED to jailbreak your phone if you want to change the visual theme of anything. If you do not jailbreak, you cannot have a custom theme.
5. Jailbreaking allows you to download apps that aren't in the app store. However, most of the apps are useless. There are a few gems... but most are not worth your time.
6. With a jailbroken iPhone, you can still access and download from the app store through the app on the phone. However, do not ever try to sync via iTunes if you have any "illegal apps" as iTunes will delete them instantly.
7. If you jailbreak your phone, you cannot upgrade your firmware without an update to the jailbreak... sometimes it takes months for a jailbreak update to come out.
Make sure you carefully consider both the pros and cons of jailbreaking your iPhone. Personally, jailbreaking my phone was the right thing for me to do. It allows me to record video (I don't have a 3G S, so this is a big deal to me), have a custom theme, set and download custom ringtones quick and easily and gave me more "advanced" options in my iPhones settings.
If you have a 3G S, I'd say don't bother unless customization is important. However, if you have a plain ol' 3G, I'd say go for it AFTER you think carefully about the pros and cons.