I have a six tap keezer with "real" kegs in it, but I also have a bunch of TAD units that I started with. I still use them for small experiment batches, and for that, they are extremely useful. They fit in normal refrigerators, or even on shelves in my garage during winter.
They absolutely will force carbonate. A full CO2 cartridge is exhausted for that. It takes about 5-7 days. Afterwards, you can serve for a couple days and then you'll need a second cartridge to continue serving with proper carbonation. Interestingly, if you brew English beers that stylistically fit with lower carbonation, the TAD limping along on its first cartridge is a nice match.
They are cheaper than kegs for sure - but once you make the commitment to really get into brewing and serving, they are easily outgrown. Still, I think they have a place in the home brewery and I use them.