I am currently running Reason 3.0 on a Mac Book Pro with a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. I used to run it on a Hewlett Packard lap top with a 3.0 Ghz processor and 2 GB of RAM. I have been able to run this smoothly on both systems both as a stand alone and running it through Pro Tools LE via rewire. Reason 3.0 has run smoothly since I installed it.
I've been using Reason 3.0 for about three or four years. I use it for programming drum beats to use as a scratch track. I use it with an M-Audio Keystation 49e to play the virtual instrument in the sequencer. I mostly use the redrum and looper sections of this program, but the synthesizers also sound great. I usually put the drum beat back into Pro Tools and record acoustic instruments over it. Reason is a powerful piece of software with a lot of capabilities. I don't know what other software I would compare this to other than Apple Logic as they are somewhat similar. I like both programs for different reasons and both work great in a number of different situations. I love using Reason 3.0 and hope I am able to upgrade to Reason 4 as I have heard great things about it!