Everyone is going to have a different opinion. Here's mine.
We started off with a Hix press almost 11 years ago. It was difficult to operate and had cold spots on the platen. I seem to remember the fuse popping a few times also.
After that, we have gone through a number of Geo Knight presses (combo, swing away and air operated) and they have been great. We had one problem with the handle on an early combo press. I happened to be travelling near their office and called them about a replacement. They invited me in, gave me a updated handle and showed me the new presses. I ended up buying another press from them that week which is still in use today. Very nice people in the office and great tech support on the phone.
Opaque transfers just plain stink. Everyone feels the need to try them, but few believe they are good enough to sell.
Would an air press assist in the application of an opaque transfer? The correct answer would be "Yes, an air operated press will help in the application of ANY transfer as it precisely controls the three things necessary for a transfer - heat, pressure and time."
Would an air press keep your opaque transfers from lifting off the shirt? Absolutely not. The opaque transfer is not as flexible as the shirt and the adhesive is very weak. If you stretch the shirt (through normal use or washing), the non-flexible opaque backing will eventually pull loose.
What features would you definitely not pass up on if you were to get a new one?
None!
...but of course you can only get what you can afford.
If you're just starting off, Geo Knight's combo press is great. It can handle shirts, hats, cubes, plates, etc. with just one press.
If you're going to be pressing a large number of shirts, you'll never regret getting an air operated press. If you're thinking about an air press, don't forget to figure in the air compressor - and a cheap pancake nailer compressor won't cut it.