Iwata Hi-Line HP-BH

Information

Necessity for fine details.
Desire for more control (the MAC Valve provides it)
Has experience airbrushing.
A beginner CAN learn on the HP-BH, but I'll still try to steer them away from it unless they really, really want it. The MAC Valve gets in the way of the early learning process.
The smaller cup than the HP-CH is nice for bigger, meatier hands (like mine).

The MAC Valve

It's like a pressure 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.
Small Cup

The smaller cup is nice because you can still fit quite a bit of paint into the cup. Enough to spray a background or cover a face with foundation. But it's not so big that it'll start to interfere with operation. The small cup is nice and sleek, though not quite as sleek and space saving as 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-BH's place is for people who generally use small amounts of paint but occasionally need more. Makeup artists and fingernail artists generally prefer the HP-BH over HP-CH. Fine artists also like the fact that they can finer details a little bit easier with the BH instead of the CH.
The MAC Valve is awesome

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-B+ 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

Unfortunately 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!
Getting Careless

Doesn't sound like a problem right? But the HP-BH can hold enough paint to ruin your work if you get a little wild and crazy while you spray (again like me). So keep it mellow!

Should your mellowness fail, try going through these instructions to remove the broken threads.