One of the things that looks interesting to me is the aggregation function. I currently have multiple email accounts and I have to have 3 (or more) browser tabs open to deal with them all. It would be very handy to be able to go to one place and get all of my mail.
One downside I can see with this is making sure that I'm responding to the correct message from the correct email address. Mixing up work and home accounts would not be good.
I needed this a few years ago badly. I ended up forwarding all email to my gmail. Then... And this is considered excessive by some... I forwarded all those to a MobileMe account and check it all in Apple Mail. It automatically detects what email address my email came to and when I reply that's the email it will show from. Works great and haven't changed my setup since.
From there you can create rules and whatever else you need to.
One downside I can see with this is making sure that I'm responding to the correct message from the correct email address. Mixing up work and home accounts would not be good.