The V64jr was the successor to the Doctor V64, both manufactured by Bung. It is a simple device, shaped just like a N64 cartridge. It has a slot in it for the boot cartridge, a connection for PC link, some EDO DRAM inside, and an Altera ASIC. The memory is backed up by 6 AA batteries and can last up to 8 hours or so without N64 power.
The V64jr's main advantage at the time it was being manufactured (Late 1999) was its price. For $179 you would have the only device capable of playing 512Mbit carts. (Z64, V64 and CD64 can back them up but not play them.) In addition, it's the most portable backup unit, as it's basically just a bigger and taller N64 cart. Since the unit does not have a BIOS, you are not constrained as to what you load onto it. Using the excellent EvekII by WT_Riker/Obsidian, you can load a ROM and simultaneously crack it, PAL-fix it, load your previous save, and retrieve the save when you're done.
However, the drawbacks start to mount when we examine the V64jr closely.
Again, like the Z64, the V64jr's main drawback is by design. As the Z64 was designed to be used completely without a computer, the V64jr requires a computer. This can be inconvenient if your computer and N64 room are nowhere near each other.
Another annoyance with the V64jr is load time. Since you dont have the choice to load from media such as CD or Zip, you have to wait for the parallel transfer on EVERY load. This gets old quickly, especially when you're trying different patches to try to get a certain 256Mbit game to run. V64jr is definitely not the most optimal unit for playing games on, and this is where the CD64 has an advantage: it has nice dev features, but is also very usable for gaming time.
Also, due to the V64jr's cost-cutting design , we have the same sort of problem that the CD64 related to passive components. The unit has been badly designed, giving not exactly 3.3v to the SIMM memory module. This gives instability and most of the time is the cause of corrupted dumps of games. Under "Downloads" you can find a PDF document that tells you how to modify it to improve stability if you have some patience and some electronic skill.