Gyakuten Kenji 2 is widely considered to be one of the best games, if not the best, game in the franchise, main games included. Dai Gyakuten Saiban was recieved well except for the ending, which left a ton of unresolved plot holes (which has been promised to be filled in this game). Professor Layton vs Ace Attorney is pretty split on opinion I feel, while Gyakuten Kenji is the only one that isn't very popular. That's my take at least.
Edit: of course this is a sales thread and I'm aware of that, just painting a bigger picture. I'm personally afraid that DGS2 will bomb to such an extent we'll never get any more spin-offs released.