Reason is about as good as it gets so far as sequencing goes. I have Reason 2.5 and I'm very happy with it. It's got tons of features and there are more expansions available freely online. If you don't know how it works, Reason is like a big synth emulator. It has a rack and you select all sorts of machines to put on it (like mixers, drum machines, delays, etc.) and then you wire them together however you like. It then plays your sequenced tracks according to your setup. Pretty much everything can be sequenced, from the audio coming from the synths and samplers to the line levels and other settings.
It's by far the most comprehensive sequencing suite I've ever played with. The only thing that you could say is missing is sound editing tools, but that's not really what this program is about anyways. (Actually, for all I know, they could be there; I still don't know how to work half of the features.) My only real complaint about Reason is that it borders on overwhelming. Also, while composing with Reason works well, in 2.5 there's no way to write music on a staff; instead it's a big spreadsheet-type thing that lists all the available octaves. I think it's probably a more practical way of presenting the information, but I learned to compose longhand so I like composition programs that make my stuff look like sheet music - I'm just anacronistic.
~Joe