Others say...

"Roomba 410"
I love my Roomba 410 to death.
This is one of the cheaper version, but it fits my particular situation. A small apartment, tile floor, uncluttered. Under those conditions, this thing works great. I run it once a day, and my floor is cleaner than it's ever been. It can't reach edges and so it leaves a tiny strip around edges, so every once in a while I run the fine-tipped suction tube of my old carpet cleaner around all the edges to get that.
The bottom line is that is now easy to have an apartment with clean floors. Put the Roomba down, it does its thing for an hour and quits, pick it up and clean it, put it back on the charger and you're done. Cleaning requires a grand total of three minutes, so whereas I once vacuumed once a month, I now vacuum once a day.
And my floors are clean, and they STAY that way.
I have some extension cords down but it NEVER gets stuck on cords. It'll drag the cords around but it has never gotten stuck. It's amazing to watch this thing go into a tight spot, bounce around cleaning, and come right back out.(I've done some programming. The program in this thing has got to be VERY sophisticated.) It gets under beds, tables, and chest of drawers, no problem. I don't have any rugs so can't say anything about how it handles rugs.
I have two cats. This thing picks up cat hair by the ton. The cats get used to it in about a week.
Replacement batteries appear to be expensive, but I've already got a line on a place that reconditions batteries, and the present battery will probably last until the warranty expires.
For my small apartment (around 500 square feet) the seven-hour-to-charge battery is as effective as the three-hour-charge battery, because it can all gets done on one charge. Besides, as a chemist I've got a feeling the seven-hour battery is likely to last longer.
Initially, the first three or four days, expect this thing to pick up a ton of dirt. In fact, on first use I recommend stopping it halfway through the run and emptying it. With daily use, the normal load will drop to a little dirt and cat hair. Depending on traffic in the room(kids, you know.I have no kids.), of course.
I believe the next cheaper model does NOT have the dirt sensor. (The dirt sensor is a thing that detects it has hit a particularly dirty spot. When this happens, the Roomba will stop and pay particular attention to that spot. It'll flash a little blue light in outraged indignation while it circles around cleaning that spot.) I recommend getting the dirt sensor (which my Roomba has). In my opinion, a Roomba is much more efficient when it can detect it has hit a dirty spot.
I already know I want one those Scooba thingies, too.

"Watch Roomba clean for you!"
I know Roomba has been around for a couple of years and this is an entry level model. It does an amazing job! It will run around the house and clean while you watch TV or just sit around and watch Roomba cleaning! I love it! However, if you have a little more cash to spare, go with a higher model that has something called a home base. This one you have to plug it in to charge and unplug it and push the buttons so that it can start working. If you have a big house or your place is full of dust/hair (like mine), it might run out of battery mid way and get stuck. The one with a home base is smart enough to go back to the base to recharge before it runs out of battery. (If you played laser tag before, it's a similar concept - fight - go back to base to charge - and go out to fight more till the battle is over)
Either way, get a Roomba - this basic model or a fancier model with a home base! :)

"Excellent robot vacuum"
This is the fourth roomba that I've owned and I love them. Even this most basic model expertly picks up dog hair around the house. I run this vacuum at least 3 days a week and it is perfect for cleaning up while I am out. No complaints.

"Awesome device! Get one!"
First off, I want to say - get this product through Amazon directly. The third party resellers don't appear to stock product inventory and it can take weeks to ship (had to cancel a previous order from a third party reseller for this reason).

As for the iRobot Roomba 410:

Awesome little device. I use it downstairs in my kitchen and living area. I have two cats and a stained concrete floor and it does a magnificent job picking up cat hair and other miscellaneous dirt, bugs, etc. The first time I ran it, after only 20 minutes, I stopped it and opened the dust bin - pulled out a massive ball of cat hair and dust I wasn't even aware was on the floor. Over time, the amount of debris I have extracted from the unit has decreased substantially indicating it is doing its job. Fantastic - exactly why I bought it!

The cats love it; they follow it around and jump out of the way when it decides to suddenly turn in their direction.

Like any mechanical device, there are a few quirks and things to be aware of:

1.) It can get hung on power cords and rug edges. Watch the unit for the first few times you run it and (if possible) modify your space to eliminate these hazards (I taped down my rugs in a few places which helped a lot). Afterwards, you can let it run without worry.
2.) It has a small dust bin and as such, may need to be stopped and emptied every 20 minutes or so (at least on the first few runs, or until the room starts to become clean).
3.) Let it run until the battery goes down (about 45-60 minutes). This maximizes battery duration and performance. Accept the fact that the battery will expire in a year or so (maybe less) and you'll have to buy a replacement from iRobot (50 bucks, big deal).
4.) The unit is made almost entirely of plastic. I estimate a life expectancy of approximately 4-5 years, maybe longer if I clean it regularly and replace the brushes, bearings, etc. when needed (all available for order in iRobot's website). By that time, I'm sure iRobot will have 4th or maybe 5th generation Roombas available and I'll definitely be in the market for a new one!
5.) Don't expect it to return to its charging base every time. In my case it usually doesn't. I just pick it up and set it on the charger (maybe one day it will find it on its own and surprise me).

Overall, I'd give it a 9 out of 10. It efficiently does the job it is designed for, has a great "coolness" factor and it is a heck of a lot of fun to watch!


"AWESOME"
I bought a refurb model of this and it is AWESOME. Picks up hair from 2 dogs. The only problem I have had is the dirt drawer sometimes comes off so have taken to taping it on and now it is PERFECT! LOVE IT!