I've got the package from this lot, and would recommend it for the £30 quid. It installs very simply and it's then easy to look at diagnostics.
http://bmtechnic.co.uk/
For example when I had an ABS fault, I looked at the code and found it wasn't something easily fixable myself (if it had been a wheel sensor, it's a pretty easy driveway job), so off to the dealer it went.
It also lets you change the car/key memory settings.
I think it only gets dodgy if you try to reprogram any of the modules, then you obviously run some risk of really screwing something up if you don't know what you're doing.