I know you said you have liking for the Roomba hoovers but have you looked in to Vielda? I brought this model a while ago and it's really good. My review of it so far.
I really didnt think this would work. I have a cat and a dog and I could easily hoover pet hairs twice a day....(although I never could be bothered, and hence got used to tumbleweed hairs greeting me in the hall every time I came home). This is absolutely brilliant for maintenance, keeping things presentable...yes, you will have to hoover the skirting and corners yourself once in a while, yes, it will snag on rug fringes, or occasionally get stuck and need rescued, no, its not suitable for deep pile carpets, and yes, you will have to do the stairs yourself.....but, oh my God, for a lazy person who has laminate/tiled/vinyl floors or short pile carpets....its bloody brilliant. I cant understand the negative reviews that cite 'problems' such as I just listed...they arent problems if you dont expect it to do things it never claimed to do in the first place!!
I wanted a roomba, but there was no way I was paying several hundred pounds for what, essentially seemed to me, to be a lazy assed gadget. I bought this with birthday money, as a present to myself, as it was fairly cheap and I couldnt think of anything I really wanted. I would actually buy it again if it broke.
In terms of how powerful it is.....I really bought it for my hardwood floors, but one day it got itself stuck in a small circle on a mat I have at the door that I forgot to lift, and when I rescued it, it was almost embarrassing how clean the wee circle it did was compared to the rest of the mat.
Another point....in a second-hand way, it nearly makes you be a bit tidier anyway, as you are more inclined to clear the floors of stuff it might get caught in...I had to rescue it the other day as it managed to suck up almost an entire block of post-it notes I had missed. My regular vacuum wouldnt have lifted that....not that it should....but the point is, there must be some good suction there!
I know it doesnt have the advanced features of, say, cleverly returning itself to the charging station when its running low...but.....to be honest, Im not sure you need to pay a hundred quid more to do that.....it does my hall, kitchen, and large living room easily on one charge, and Im always here when its on...I dont go out and leave it becasue it DOES get caught in things and need rescued....and, frankly, if youre the sort of person that wants to go out to work and come home to a clean house, then you probably need a maid, not a robotic hoover.
If you want the moon on a stick, you'll be disappointed. If you expect what it says it will do, but are a bit skeptical, you'll be very pleasantly surprised!!