I've owned the Roomba 555 for over four months now. I delayed writing a review because I wanted to see how reliable it was.
I bought the Roomba when I was recovered from a bad bout of flu and hadn't been well enough to vacuum for six weeks. I was desperate. With a lot of small rooms, fairly crowded with furniture, I didn't expect it to do much other than clean the centre of the rooms. In fact, the cunning side brush meant it was surprisingly good along skirting boards and around furniture. I move the odd item, like wastepaper baskets on to settees, and just let it loose. There are only a couple of spots it can't reach, like behind the toilet, and of course it can't do the stairs.
Initially, it needed its brushes cleaning a lot, but that stopped after it caught up with the cleaning backlog. It's pretty reliable at tracking down its docking station to plug itself in to recharge power,even when working its way round several rooms. Since I'm paranoid, I normally pop something across the top of the stairs when it vacuums upstairs rather than trusting the sensor, but the couple of times I forget it didn't plummet over the edge. I haven't bothered trying the timer since I usually run it when I'm in.
It cleans well on ceramic floor tiles, thin carpet tiles, and thick pile carpet, and seems to have cheered up the rather flattened pile of the hall carpet. It takes its time doing several circuits of a room before declaring it clean, but time is no issue when you don't have to stay and watch it except for entertainment value. It's quite amusing when it spots a dirty spot, the blue light flashes, and it does an extra circle to clean up.
I empty it every time I use it, because it only takes two seconds to tip out the dust. That's very quick and easy, which is important because house dust isn't good for me. I've bought spare filters and side brushes but haven't needed to use them yet.
I always make sure it's fully charged after use. If its two weeks since it was last used it seems to have lost a little of the charge, but a week seems to be fine.
If anything, the carpet is cleaner than when I used a conventional vacuum cleaner. I've given up using the old vacuum cleaners, and just bought a small handheld vacuum cleaner to do the stairs and behind the toilet.
A quick update on this. I've now been using it for about ten months. Still fine. Only changed the filter once.