I've used and actually owned and subsequently rented the Spa-in-a-Box. Personally I think it's the best choice out there for several reasons. The sides are totally strong - it's no problem to sit or lean on them (I did plenty of leaning over the sides of the tub). It doesn't feel hard (granted mine was on a carpeted floor both times), there's plenty of room to move around in during labor (I sort of would pace back and forth like a cat I guess) and it's plenty roomy for someone else to be in there to catch, if you want (my dh did), it not only maintains the heat of the water but can heat it too if needed, I love that it has the soft locking cover (to keep pets/kids out and heat in), the jets are fine although I've never used them *during* labor and haven't known others to do that but I guess that's just personal preference, and it was wonderful to relax in it in the days/weeks leading up to my births. Oh, and the depth is fantastic.
Mama to four remarkable kiddos, all born at home.