How far does the BYP-transmitter reach?

About 100-200 metres (diameter). But it depends on the thickness of your walls, windows etc. You can link up several transmitters to have a bigger reach. You'd set one transmitter at a certain frequency. Let a radio receive the signal 100 metres further, and attach it to a transmitter that transmits at another frequency, which can be picked up by a 2nd radio, attached to another transmitter. etc.etc.etc.

Is it legal?

Not entirely. In most countries the use of FM (and AM) is forbidden. However the transmitters are not that strong that they take over existing radiostations frequencies. And why should the distribution of content offline through radiowaves not be legal?

What about the itrip?

Yes. The itrip mainly does the same as BYP. However the itrip does not offer you a manual on how to make your own, it doesn't reach as far as BYP (10 vs 100 metres) and it also doesn't put forward the idea of broadcasting to anybody but yourself, the itrip costs about 40$, that's 7 times more than the BYP, and the BYP unit works on any musicplayer, not just the ipod.
Some say the BYP unit is prettier than the itrip. but that's a matter of taste.

I want more questions & answers

Great. You can put your questions or read more answers in the special questions thread that i made on the BYP-forum.