I was conned into buying one of these plus 3GB/3 months of Three PAYG coverage.
It's almost unusably slow, even using stuff like OnSpeed, in an area where Three say I should get full 3G. My iPhone gives me four bars and full 3G browsing but I get - at best - one bar on my Mac and speed akin to 28.8 kbit/sec dialup.
The Three supplied software is decent enough though it works by auto-playing from the internal USB read-only memory whenever you plug it in so you get a Finder window each time it is plugged in, instead of the software that you've already installed coming up. This behaviour can't be changed or doesn't seem to want to. No SMS capability on Mac either.
In short, the modem is probably as described above, perfectly servicable with an antenna socket to boot (tho antennas not sold in Three shop, only onine at niche retailers), but be VERY VERY careful about what network you use as Three are to be avoided for data at reasonable speeds, even in full HSDPA coverage areas.