The Roomba Discovery is a Discontinued Roomba model from iRobot. The Roomba Discovery added a larger dust bin, a Home Base that automatically docked the Roomba to charge, improved firmware, and a dirt detect feature. All second generation Roombas are functionally identical, though some have more or fewer buttons, accessories, or casings, and all featured updated programming after mid 2005.
In my experience, the iRobot Discovery is great... on carpets. On Carpets, it goes slow (but efficient) and picks up tons of dust. On Hardwood Floors, it goes to fast and doesn't pick up anything. (I think size has a slight impact) We have a fairly big upstairs (3 bed rooms, and a gathering area) and it does great. Sometimes it'll be stuck in a room and you need to pick it up and move it out. Don't leave anything on the floor. (Especially cords. If you have any, you could try taping them to the walls, or placing a virtual wall in front. Or try this from a RobotReviews user "Eco-friendly "Virtual Fence") If there are cords, it will get tangled and either deplete it's battery which results in a wasted day, burn the motor out, or both. I've experienced it all. The newer robots to not act in the same way, as they have an auto-untangler feature. (Thats a mouth full)
Some of the older Roomba Discovery's have older firmware. The new firmware added
You can identify the firmware using this post from RobotReviews.com User, Howard:
Disconnect the roomba from the charger.
Hold down the power button for about 10 seconds.
Count the dits and dahs it is base 5 just like the test numbers.
Each short beep is 1 & each long beep counts as 5.
Its in the format year month day.
A #4210 with a mfg-date of 041014 and got .... - .... ---- . meaning 040921
A #4100 with a mfg-date of 041023 produced the same .... - .... ---- . meaning 040921 again.
Upgraded the 4100 with a scheduler remote mfg on 050929 and got - .... ---- . meaning 050421.
If you are on the old firmware, there are many ways to update. One way is through the Scheduler Remote. Another is through this service: http://www.protechrobotics.com/roomba-repair-service.php?cat=40
The Roomba Discovery is definitely still great. However, it does seem to get stuck on wires often. If its at a yard sale, it's definitely worth it.