Mike, I've not used one, but I've seen a partial exploded view of at least one, and having worked on a number of other air tools, I'll give you an "unbiased opinion" /forums/images/graemlins/wink.gif
1) Look at what the specs call for in air consumption - do you have enough compressor?
2) Does it come with a manual with an exploded view and parts list? Most air tools are pretty reliable if you don't get dirt in them and do keep them lubricated.
3) Is it a company that's been around long enough to have parts readily available?
4) Most air tools are relatively easy to repair if they have a decent manual and parts availability.
5) Does the company have a "customer service" phone number? Sometimes the "instructions" in the manuals are not quite adequate. Example: To rebuild an IR231 impact wrench, the manuals show to pull the air motor out of the housing. What it doesn't tell you, or even mention, is that you must not pull the cylinder out separately; you must pull it out as one unit and in at least two-thirds of them you must heat the housing with a torch or you'll never get it out.
Now I've a bit of experience with those manual post hole drivers and if I had very many posts to drive, I'd definitely be looking for a pnuematic post driver. /forums/images/graemlins/tongue.gif