I'll also chime in with a

for the Daytona Twin Scan II+. I bought it last year and used it to do my tuning. hd-dude is correct that it is more difficult to get good consistent runs (sweeps) than you can on a dyno. The device does allow tinkerers, like myself, to tinker on demand.
The best modification I made to the device was to wire it into the Acc. switch so I could start and stop the recording at will. With only 3,600 data points of data available and a fast recroding rate (5 points/sec) for doing 'pulls', the data buffer fills up in 12 minutes. Unless you can toggle the recording on and off, you wind up with a little bit of planned test data and a whole lot of not so interesting information.
This device isn't for everyone, but I like it a lot, and now that I've got a trailer I can use as a weighted sled to slow down the acceleration, I should be able to get better results. If the trailer drag isn't enough, maybe a drag chute would help.

