The only downside I can see about it is that is made by the Kazaa people, so there is probably a temptation for them to include spyware as they did with Kazaa.
Although according to Davey Winder in this month's PC-Pro magazine, it doesn't contain any spyware right now.
Aparrently more than 4 million people have downloaded already and there are more than 125,000 users online.