Yeah using 3G dongles can be sporadic, I get pretty poor reception here but I've noticed times when I get the download speeds just under 200kbs (whats that roughly about the same as a 1-2Meg connection?), theres no reason then if he in a good reception area most online games are completely playable.
I remember reading a magazine that was testing out 3G dongles and laptops last year to see if you could do proper mobile online gaming. I think they only found 1 or to locations (in/around the Bristol area I think) that they got good enough speeds.
True never tried to game with my 3G so I cant comment on the latency personally but surely if there is a good constant signal it should be fine. Either way though the magazine article (think it was PC Gamer UK) proved that it is possible in certain places, though not advisable with the current state of UK coverage.
We use to have a Vodafone dongle, was an absolute nightmare, was fine for downloading stuff on live, but playing online was a nightmare, use to get kicked from games all the time, think it had something to do with the NAT settings, they were set to strict with the dongle and caused problems all the time.
Best of luck, when I used one last year I had terrible times using a T-Mobile dongle on my 360. Sometimes is worked ok, other times it wouldn't find any servers at all and was so slow I just quit. GTA4 gave me no end of errors, it would join a server then disconnect, it was due to the strict NAT settings on the dongle, maybe they have sorted that out now and it will be ok.
My dad uses one of these as he needs it on the go. Im might impressed with the connection.. generally its alot better than his home virgin media connection which gets throttled really badly after about 4pm.. till midnight.