I went twice in the same week from Milan. The first time I went on the organised tour bus at a cost of €20. I bought the wrong size trousers for my husband so we thought we'd return and exchange them. I'd have got a25%discount on a second bus tour there but we thought it might be fun to hire a car. Well it was not, and I don't recommend it. Cost wise it is really expensive when you take into account GPS hire, Swiss road tax of something like 35 SF just to cross into Switzerland for the day and about 4 road tolls in total in Italy. Then you have to deal with traffic going in and out of Milan. We also decided a drive around Lake Como might be lovely. We didn't anticipate the narrow roads and the Italians who don't like slow cautious drivers. The bus is the best option to the outlet store...... take it.
The excellent designer stores were not super cheap, but items were discounted to make them maybe 'just' affordable. I would have liked to be there during July or January for the big discounts. We bought quite a lot, but I was left with the feeling that Swiss stores were dabbling in more than clothes and accesories they were foreign exchange traders too.