I've got a TomTom One and bought the SE USA maps for it, not great but certainly handy if you have no idea where to go
The Kissimmee/Orlando area is rapidly developing, the current maps are not up to date, on our November trip it didn't know where the sub-division I was staying on was, the road going past it was on but that was all, luckily I looked on Google Maps and MultiMap and found it on there and added it as a favourite manually. Directions are not always the best/easiest/shortest route, coming up I-Drive South frpm the 192 going to Seaworld it repeatedly told me to turn left onto Vineland and then drive south down to the I-4 and come back north and then come off on the Central Florida Parkway, luckily I knew that Seaworld was about 1/4 mile straight up and then 100yds left on the Central Florida Parkway. I have connected to TomTom to update the map but it still says there is no update for it after buying it a year ago.
You'll need a larger memory card if you want both the UK and SE USA maps stored, mine came with a measly 128Mb card, both maps take approx: 400Mb of space. Luckily 512Mb or 1Gb SD cards are very cheap here (cheaper than in the USA).