There were 3 main backup units for the N64. The Doctor64, CD64, and the Z64. Both the Doctor64 and CD64 used CD-Rs which could hold about 50 N64 roms per disk and you would select the game you wanted to play with the units own built in OS, while the Z64 could hold about 12 or less games on zip disks. It was easier on the Z64 to backup games as it would just copy the cart to the zip drive, while both the D64 and CD64 had to have extra hardware to connect to the computer to transfer carts for backup purposes.
For homebrew development, I believe the CD64 and Z64 were easier to make your own games/programs on. I'm not sure now, but in the hayday of N64 Backup units, the Doctor64 was the most expensive at around $300 i think and Z64 was pretty close to that too. The CD64 was the cheapest of the bunch, and it showed in the first generation of units (horrible Honk Kong manufacturing). They have since fixed the problems though with the 2nd generation.
Towards the end of the N64's life, the company responsible for the Doctor64, release the D64 Jr, which was a small cart like device that had to be connected to the computer to transfer roms to the device, but it had the benefit of being really cheap (about $70) and it was the only unit with 64Mb of ram which was required for games like Resident Evil and Conker's Bad Fur day, which are unplayable on the other 3 units because they only have 32Mb's of ram and are not upgradable.
I personally had the CD64 1st Generation, and after i solved the problems i had with it, it worked great and was fun to try the demo's and other homebrew games/applications on the device. I remember playing the german version of Turok: Dinosaur Hunter and seeing that they removed all the dinosaurs and replaced them with robots. It was the wierdest thing i've seen in a game.