I can speak from experience, but don't assume that I like it... 
If you are going to go shoulder holster and the gun has a barrel of 5 inches or less in length, go horizonal. If it has a longer barrel, then go vertical. Longer than 5 inch barrels will protrude out past our body and print from the rear, unless you are an extremely large person. Horizontal also has a more natural grip, especially with autos and they seem to 'hang' more comfortably under the arm when doing any activity past just sitting there....
Now, with that said, let's talk about the plus/minus of shoulder holster carry...
Plus:
The 'CDI' (Chicks Dig It) factor is high.
The comfort level can be good with the right holster as long as the gun isn't real heavy.
The seated draw is very easy and fast.
Allows for concealment of a larger handgun than would otherwise be capable in other form's of carry (think Dirty Harry and his 6 in model 29 .44 mag)
Usually allows for spare ammo to be carried on the 'off side'.
The amount of adjustment built into most modern shoulder holsters is great, allowing for a very customized fit. (once you figure out all the straps, loops and chicago screws)
Minus:
You have to wear a coat/jacket to conceal... Unless you live in Nome or portions of Maine, this is not a year 'round CCW solution.
If covered with a zippered/buttoned coat/jacket, the draw can be slow....
If carrying a heavy gun, the comfort factor can be severly decreased as the weight will give you a sore neck/back in short order.
Re-holstering requires two hands. (this is often under appreciated, but until you have had to holster while going 'hands on' with some sh*t-bag, whom hasn't raised the bar high enough to allow you to shoot him, you can't really appreciate how important being able to holster with one hand actually is. Trust me on this one, I have the scar to prove it.)
When carried with spare ammo, they can get heavy fast. (See above)
They are adjustable six ways from sunday.... this can cause you to go mad trying to get the gun to wear 'just right'.
IF I were to recommend a shoulder holster (not saying I do, but IF), I had my best experience with the Galco Miami Classic. It was the most comfortable and broke in well. They are not cheap, but quality rarely is.
Just my dos centavos... Remember, my opinion is worth every penny you paid for it.
-Tac