Is HSPA+ active in your area? The download speeds are quite impressive for AT&T :-)For what it's worth, I signed up for a month of AT&T postpaid service without a contract, to compare it to H2O (on a Veer).
H2O gives me ~800kbps down/ 250kbps up today. AT&T gives ~5.0mbps down / 600kbps up.
Both are faster than my Sprint Pre 2 lately, which is about 500 down/ 150 up.
However, I haven't used the AT&T SIM yet for more than just basic testing from my apartment. Personally, I was having issues with the H2O SIM where despite the decent benchmarks for data, in practice it was horrible. It would take a minute to switch to the next track in Pandora, I couldn't get Google voice to work because the webpage wouldn't load without refreshing it 3 or 4 times, etc. Of course your results will vary, but the H2O SIM just wasn't stable enough for me to actually keep at it.