If you have an Acer, you've probably noticed the dearth of serious cases on the market for it. The number one cause of netbook death is a cracked screen, followed by hard drive failure. WIthout a decent hardside padded case, one or the other will happen much easier! Without a second thought, the only case I wanted for mine was a Brain Cell.
The Acer 8.9 fits in the Size 7 Brain Cell. However, the 10.1 won't - it will slide into the case, but the ends will bow out and the top won't close if you have an extended 6 or 9 cell battery attached. My Acer's actual model number is AOD150. The netbook actually measures 10.3 x 8.3 x1.3, but in Acer's fabric case it ends up 10.5 x 8.5 x 1.5. In a Size 7 I'd make very sure not to drop it!
Yesterday I tried my Acer into a Size 5, and it works perfectly, fabric case on or off. Sure, it's a half inch wider than the netbook is, but that means I don't have to remove my wireless mouse and Bluetooth mini-dongles from the USB ports. If I do remove the dongles, the brick half of the power supply slides down the side of the netbook with no trouble at all. The heavy wall cord half, alas, needs to go in the outside pocket.
The other nice thing about getting a slightly larger Cell is the slightly more than 1" space above the netbook under the flaps. It's more than enough space to stow a second extended battery! Or a wrist rest. There's also just enough space to slide a file folder into the Cell alongside the netbook, for safe and crumple-free transport.
The 5 may not look as appealingly small as other cases, but it works better, IMHO. Note this only applies to the 10.1" models; if you have the new 11.6" screen model (AO751h), the 5 MAY fit. You won't be able to leave your dongles attached - the width is listed as 11.2, and the cell's length is listed as 11.3. The 3-cell battery will fit, no problem, and same with the 6-cell. The real problem will be the new length of the unit, since it's actually narrower height and depth wise.