Do it if -
- you have nothing on your floors
- you don't mind a meticulous cleaning of the robot vacuum fairly frequently
- you know it will only do a partial job and you will still need to vacuum about as often as now
- you know it will get stuck and you will have to rescue it
- you know it makes noise at sometimes inconvenient times
- you know it as a lot of parts which need periodic cleaning or replacement
- you have extra cash for it and no better use for the money
As a counter:
-There are animal toys on my floor and I don't need to pick them up unless they have strings, my robot just pushes them around.
-Mine doesn't require meticulous cleaning, I just dump the bin and replace the hepa filter when it gets dirty. It takes about 30 seconds.
-I've never meticulously cleaned the robot. I've maybe wiped it down with a wet rag a few times a year. And when DH had long hair, I would have to de-hair the brushes, which took about a minute.
-Mine does a thorough job, I have never vacuumed my floors with a regular vacuum. I do dust the baseboards periodically though.
-Mine does get stuck, and it beeps for help. Occasionally having to kick it out from under the fridge door is much less work that doing a full vacuum.
-Mine doesn't make noise unless it's running. So I pick the times it makes noise.
-I have never had to replace anything except the hepa filters, and once over several years of constant use I've replaced the little brushes because the first set got ruined by DH's long hair, which I didn't realize was tangling them because I rarely clean it or do anything to it.
-I didn't spend extra cash on it. My vacuum was old, crappy, and partially broken and I was looking to replace it. Instead, I bought a $200 robot vacuum and kept the old broken vacuum that can't do floors, but can still suction enough to clean my sofa and car, which is pretty much all I use it for.
I don't know about units other than mine, if the expensive ones need tons of maintenance, but mine does a great job on my wood and tile floors, and for may years now has required very, very minimal maintenance.
That said, I live in a small apartment with no stairs and a simple layout, and minimalist furniture, all of which can be gotten underneath by the robot, no carpet, no shedding pets, and I'm very tidy in general.
If you have any factors that make regular vacuuming necessary anyway or make the robot vacuum less useful (ex: low clearance furniture), then you aren't gaining much from having a robot on top of a regular vacuum.