Sorry haven't used skype so this is not first hand advice...
Yes it is legal.
Yes it is made by Sharman who also made Kazaa. Sharman have asserted it is free of any nasties, but they also did this with Kazaa so don't know if this is true.
Apparentley (sp) this is the future of telecommunications but I can't see BT standing for it. They must be due for a huge loss in sales so will probably increase charges to ISPs to compensate or find a way to charge Sharman. One train of thought is it is better to get on the train now, before Sharman have to start charging also.
Sorry, nothing definitive here, but I hope t helps slightly.