UMA means Unified Memory Architecture, and essentially boils down to the "shared memory" you mention. The thing is, even though you have 4Gb to play with a UMA card is still slower than one with dedicated memory.
As well as running slower (main system memory is slower than dedicated graphics RAM) it uses the same buses within the system to transfer data around. As a result, heavy graphics use will slow down the overall speed of the system.
Hard to say exactly how good or otherwise it is as it depends on the actual make of the card, motherboard and what you intend to use it for. The majority of laptops use these, though, so they're obviously not complete rubbish. Just don't expect it to play the latest games at all well. Fine for application use, though.
