This program is great. It's midi sequencer is among the best I've used. It's not better then Sonar 5 when it comes to midi editing features, but it's close, and far easier to use. An easy to use piano roll and tons of midi parameters does make it a power house. I can't comment on the groove matrix since I don't really use it, but it is a nice feature to have for those who want to trigger beats and tunes for live performance.
It's per track arpeggiator is very good. It come with tons of presets for guitar strumming and picking, bass patterns, synth patterns and others. And you can customize and design your own arpeggio patterns. It's no Triton or Karma, but it's arpeggiator is one of the best I've used in software.
Project5 V2 does have some nice useful instruments, but the best TO ME has to be the Dimension synth. I liked it so much, that I purchased Dimension Pro. Right now it is one of the most used synths in my studio, but that's for another review. Other instruments worth mentioning is the DS864, Roland Groove Synth. and PSYN II.
Project 5 V2 is compatible with vst's, dxi's, and have full support for acidized files. So far I experianced no problems when using third party plug-ins. It also supports rewire. So if you're a seasoned Reason user, Project 5 V2 is the perfect compliment to it. As I used them both religously.
Obviously, like ALL music software, there are things that Project 5 V2 can't do, or doesn't do well. For starters, it doesn't have a mixer. Now, each track does have volume and panning controls, it just doesn't have a seperate mixing console. Many of the effects it comes with range between average and above average at best, although useful for certain situations. The software can record audio, but it's not very good for wave or overall audio editing. It's best used for making short loops and maybe recording some vocal phrases here and there. Project 5 also uses a bit more CPU power then some comparable software, but it's much improved over the original Project 5 which was CPU hungry.
All in all, is best to think of Project 5 V2 as a software version of a music workstation, like a motif or a Triton...just cheaper and more flexible.