Iwata Hi-Line HP-AH

Information

Necessity for fine details.
Desire for more control (the MAC Valve provides it)
Has experience airbrushing.
A beginner CAN learn on the HP-AH, but I'll still try to steer them away from it unless they really, really want it. The MAC Valvegets in the way of the early learning process.

The MAC Valve

It's like a pressre regulator at your finger tips. The only downside here is that you don't get a pressure gauge to go with it. In effect, you have to dial in the air pressure by feel instead by the numbers.
No Cup

Yeah, it sounds silly but that cup can get in the way when you're trying to aim! It also looks sleaker and takes up a bit less space. Top notch makeup artists tend to like the HP-AH
Cut-away, pre-set handle

The cut away lets you clean out your airbrush quicker. The preset let's you limit the amount of trigger pull you can have. The preset is quite useful when you have to do the same stroke over and over and over and over.

With only a few exceptions I like all the Iwata airbrushes. They each have their place for users. The HP-AH's place is specifically for people who understand that they won't use much paint at any one time--ever! That generally includes makeup artists, fingernail artists, and fine artists interested in the smaller needle and nozzle combination.
The MAC Valve is awesome

Of course, like I mentioned earlier the External MAC Valve can duplicate the effect on most airbrushes, so it comes down to a preference for the built in equipment or the add-on. If you prefer the add-on you might want to look at the HP-A+ instead.

These Cleaning Guides links are specifically the ones you need to clean this particular airbrush.

No Place to Set it Down

This is an unfortunate oversight. Somebody needed to include a kickstand! I know many airbrush compressors come with an airbrush holder and there are many manufacturers that make holders of some kind. In fact, you can even make your own with some creative ingenuity.
Tip Dry

Unfortunatly EVERYBODY has to deal with tip dry if they use airbrushes. Though one of my favorite methods to eliminate tip dry is to use a slow reducer (the paint won't dry as quickly and therefore won't stick to needle as much. Otherwise you'll need to check out the How to Remove Tip Dry methods in the cleaning guide section.
Bent Needles

If you take the needle cap off to try and get finer details you WILL bend your needle tip at some point. I usually suggest a Crown Cap. Then you can still get the detail but your needle (and possibly the person you're working on) stays safe.
Clogs

Clean it! Paint dislikes being sprayed and will do it's best to punish you for your outrageous actions! BUt don't worry, the cleaning guides will help you through these predicaments.
Breaking the Nozzle Threads Off Inside the Airbrush

You are far stronger than the nozzle. Be gentle when twist it in and out!

Try going through these instructions to remove the broken threads if you get careless and break it anyways.