3G basically makes improvements over the previous generation of data transfer rates. In the case of laptop cards (with AT&T), EDGE network gets you at most 3 times the speed of dial-up (between 80-160kbps) while 3G allows an average of 400-700kbps, with bursts of up to 1.4mbps (Depending on your signal strength, of course). Not sure of the exact speed increases for cell phones, but it's noticeable, especially if you have an iPhone 3G.
The next step up from 3G is HSPA, which is currently in a couple major metropolitan areas. It's my belief the new iPhone coming out will have that sort of connection capability.
There are also some talks about improving the existing 3G network, I believe it's called "3GPP2". It would run on the same 3G phones that are currently out, with faster speeds for data transfer.
Some companies are talking about working on a form of "4G", namely Sprint, if I recall...reviving the "WiMax" project, but I highly doubt it's going to survive again.
So to sum up and actually answer your question:
As far as reception goes, now that we have moved to an all digital network (analog being blammed) I've noticed better reception in 3G areas, but mostly in call clarity, not so much in signal strength. The main improvements are on the data speeds.
Sorry for the long post, I've just been working for AT&T for too long, haha.
EDIT: Luke, I lol'd hard. I actually get people walking in all the time thinking it means "3gig". They actually call it that. Not to mention they call SIM cards "SIMS card".