Agree with Nrembis.
If you are not using an AI computer, the download piece is not all that important.
This is really a personal preference dependent on your geeky-ness.
I am a complete geek. Not only do I download my dive computer, but I also write in my log book (still).
I know that I will at some point have 2 AI computers. I currently use a hosed computer, but I have a transmitter for wireless and will be buying the wireless computer at some point. Downloading my individual dive computers into my laptop will give me a big picture review of all my dives.
I also write stuff into my log book. High level stats of the dive and also my comments on what I saw, enjoyed, problems, ect. I didn't know why until this month. I had my laptop stolen. EEEK. Without my paper log book, all my notes and musings would be gone. Sure, I would have the original dive logs off the dive computer still. But, the personal information would be lost. That's the best part.
All dive software is, I believe, proprietary depending on the computer manufacturer.
here is another thought - I don't know much about it, saw the vendor at a show in Chicago and I am just poking around with what they offer.
www.diversonly.com offers a web log book to keep track of your dives. I don't know if you can download/upload directly or if it is all keystroking. I believe membership is $12 a year for this service. The idea is you will be able to access your dive log, through any PC at any Resort/Dive Shop, and review your dive logs and certfications. Don't quote me, I haven't joined, still poking around in my spare time, when I am not posting here.
After my latptop was stolen, I started thinking about using more web-enabled services and remembered their service. Your question started me back down the road of to download or not. I'll send the designer/owner an email and see if he is interested in posting in this thread, without making it too much of an infomercial.