I don't have an iPhone, but I do have the iPod Touch, and I have used my friend's iPhone. I would say, hold off buying one until June. Mainly because you will probably get a better phone for cheap (if you can wait). You will either get a newer model for the same price as now, or one of the current models for cheaper. This is what I'm doing.
I think that the phone is completely worth it. Just the little bit of time I've had with my iPod Touch, I'm totally and utterly in love. This is one of the greatest devices that I have used, in a long time. I know that's a lot of praise to heap on something. But it really is an amazing device!
Usefullness - you can't sync "Notes", which is annoying, but you can sync bookmarks, iCal events, photos, etc... and that makes for a wonderful experience. I get a call (on my non iPhone) and I can use my Touch to type the event into iCal right there without having to run over to my computer. Especially useful when you are out and about, and then when you get back to your computer it will sync back with your Mac. Email is right at your fingertips, photos and so on. I have used my Touch to show photos to family numerous times over the last two weeks that I have owned the device.
Web surfing speed - This is quite slow. For instance, I went to Engadget and it took about 10-15 secs to load to a point to where I could surf on it. So depending on the app, it might be just too slow for typical web use. It is nice to have, though, when you just need to get to the web and have no other means of doing so.
I'm sure others have greater ways of expanding on the extensibility of the iPhone, but those are the uses that have been of greatest importance to me.