I got a 500 AI a few weeks ago and it's a pretty nice unit. I really like the large digits and large display and backlighting, which makes it very easy to read in all conditions. It works fine, and is easy to use.
I have the software, too, and the download function works ok but I feel it should do more. I can't add dives to the log, so I have 2 computer logs, pre & post 500AI, and I wish I could merge them. It doesn't log air consumption rate, which I would like to compare from dive to dive. But the basic function and charting is decent. I prefer the horizontal layout, with the compass on the long side, to inline units like the Oceanic.
The price, through Liesurepro, was really nice, too.
I've read about reliability problems, but mine has been fine and I am happy with it.
Aloha,
Jonathan