Personally, I like the ReadyNAS Duo a lot more than the Drobo (hard to explain, I just trust their tech better, and the ReadyNAS is natively networked, rather than needing an afterthought add-on). Last I checked, Amazon will sell you an empty ReadyNAS Duo and a couple WD Green 1TB drives for ballpark $500. That said I haven't got a ReadyNAS yet (because money has so many uses these days); I'm using my second most favorite backup setup, a 500GB laptop drive in an external bus-powered FireWire enclosure. I'm using a MacAlly PHR-S250CC enclosure (which I'm very happy with), using a drive I already had, but for a complete setup, I'd probably go with one of Other World Computing's packages for about $150. This loses RAID (which I ultimately want very much to have, for reliability), and isn't networked (which would be good for backing up multiple machines, and ease of use), but the bus-powered drive is so damned easy to use that I actually do it every day (set the drive next to my laptop and plug one cable between them, Time Machine notices the drive and starts a backup, 5-10 minutes later it's done, and I unmount the drive, unplug the cable, and put it back on the shelf).
My primary machine is a Mac; I use Time Machine for daily backups, and use SuperDuper to clone my MBP's drive onto the same backup disk every few weeks (minus a number of large directories that I know Time Machine is getting anyway); this gives me a backup drive I can boot from (via SuperDuper), and a lot of incremental history stored in a very usable manner (via Time Machine). And a backup system that I actually use because it's painless.
Add a ReadyNAS, and I could have my laptop automatically backing (hourly) up any time it's on the home network.
As far as on-line backup goes, I haven't been convinced yet. It eats a lot of bandwidth, and it means that someone else (that I don't know personally) has a copy of all my data, with only their promise of encryption keeping them honest. Sure, there isn't much there for anyone else to get worked up about (a variety of legally purchased music and software, a bunch of old email and vacation photos), but if it's not out of my hands, then that's one less thing I have to worry about. I do love DropBox for moving non-confidential files around, but I wouldn't use it for backup.