I ended up buying 3 different models through Amazon, and between using them personally and getting feedback from my students have some thoughts about what one might want to consider when looking for a case:
Kensington - KeyFolio Pro 2 Removable Keyboard, Case and Stand For iPad 4 with Retina Display, New iPad (3rd Gen) and iPad 2 (K39512US) - approx $50 AMAZON
This case offers good protection so is a solid choice. To use the iPad alone, it is easy to remove the keypad, however, the bottom half of the case will hang loose and flap which may make using the iPad alone a bit awkward.
New Trent IMP39B/NT-39B Embassy Keyboard Case for iPad 2, Bluetooth keyboard /w leather finish back cover for Apple iPad 2 3G Tablet, WIFI Model, 16GB, 32GB, 64GB, with three different adjustable angles. - approx $30 AMAZON
The iPad and Keyboard are separate which makes it easier to use the iPad alone unconstricted. The iPad back is also protected with a case [other models didn't have this]. One note, however, this is made of hard plastic. Though we didn't have any problems, it may be more susceptible to cracking or breaking if dropped. It also takes practice to get used to unlocking the case when closed. But at $30, it's a good deal.
Logitech Ultrathin Keyboard Cover Black for iPad 2 and iPad (3rd/4th generation) (920-004013) - approx $75 AMAZON
I like this case. It is light & sleek looking. But the back of the iPad is not protected & both are easily scratched. I have found that an additional sleeve is needed for full protection. Not for the classroom.
Why a Keyboard? and Just any Keyboard?
Naturally you don't have to buy a keyboard, some might even argue that if you want a keyboard just use a laptop. But as one of the areas I wanted to study is PRODUCTIVITY on tablets, I saw being able to write an important feature. Not to dismiss the argument of tablet vs. laptop, however, I do believe that iPads offer an opportunity to teach, learn, and demonstrate learning in ways that are different from lap tops. For that reason I specifically selected tablet keyboard cases that were removable so that: a) we would be able to write if needed, but b) the keyboard wouldn't get in the way of exploring the other features and uses the iPad affords. In my view, trying to use the iPad as a laptop is ultimately not possible, practical, or even desirable.
Is there a Downside?
For personal use, a keyboard is probably a useful addition, particularly if you get frustrated by how little screenspace is left once the built in keyboard is activated. Just like any computer, the keyboards are fragile and wont stand up to having things dropped on them. In a classroom setting, I would probably only use keyboards with older students for specific writing activities. This is another example of where having removable keyboards can be a good option. These can be removed and stored separately, while still leaving the device protected in it's case.
I've focused on keyboard covers but of course there are many non keyboard options. The iPad Smart Case which automatically puts the tablet to sleep with you close it is great; Otterbox offers a sturdy silicone cover, or KidBox has a handle which also turns into a stand - all might serve well in the classroom. Ultimately it depends on the age of the students and what the devices are going to be used for. The take away is that some thought should definitely be put into this before buying, especially if it is for a classroom set. See also PCMag Review Best iPad Keyboards