I went to these outlets last fall on my trip to Florence, and Gucci is the best one....wallets & shoes are on a GREAT discount, but I found their handbag selection to be lacking. The other outlets are eh, just okay. (
Prada outlet is NOT in The Mall, its in a different outlet mall that I did not visit). Everything is still pretty expensive, and on my student budget all I was able to afford was a pair of Gucci mules.
To get there, you take a bus from Florence (can't remember the exact cost, i think it was like 6 euros round trip). It took me ages to find the station...it's near the main train station but sort of like off to the side. You may want to ask for directions. The bus leaves 2 x a day, and picks up 2 x a day...if you take the first bus to the outlets & the first bus home from the outlets, I think it gives you about 2 hours shopping time, which trust me, will be enough.
In my opinion, it's worth going, just for the Gucci outlet if you are already planning a few days in Florence. If you only have limited time, it might be better spent checking out sites in Florence.