I haven't used these tuning boxes on an Alfa, but have purchased one for my Peugeot Diesel after much research so have some experience that may be useful...
Firstly, be sure what you are getting - there is some real tat on eBay, and one scam currently used is "a resistor in a box" which essentially just fools the ECU by leaning the mixture by the same amount throughout the range - not too great!!
Some of the cheaper boxes have microprocessor control but are quite dodgy and dont work so great - as with most things in life - you get what you pay for...
Having said that, I paid about £50, and could see the guy had bought the box I was looking at for almost double that a couple of months previous and had had a bit of a smash resulting in him offloading bits quickly! I found it to work relatively well, it did increase the perceived acceleration of the car but didnt impact the mpg in either way.
So, in summary - do your research and go for it, but in my opinion dont spend too much, and dont expect too much!
And remember of course - all mods should be reported to your insurance to ensure you are still legal!