I've tried several types of holsters for my H&K USP, and from IWB to small of the back, the Galco MC212 is the winner so far when it comes to comfort and concealment. I had a nylon shoulder rig, but it had a bad habit of the straps rolling up, but with the holster and mag carrier hold downs on the 212 that problem just doesn't exist. Once you get the straps adjusted it's very easy to forget you're even wearing it. Also, a lot of the shoulder holsters don't have the double tie-downs that the 212 does and some don't have any mag holders at all, and that's one of the things that make the 212 comfortable. When you have a fully loaded firearm hanging on one side of your body, with nothing on the other side to compensate for the weight, it can be more of an aggravation to wear it than it's worth. With the 212 fitted on you right, it hangs just far enough back that you can throw on a long-tailed shirt, without even buttoning it, and unless you happen to get hit with a big gust of wind or someone grabs you, concealment is no problem at all. Of course here in Texas, with the terrible winters we have, I'm usually already wearing a tee-shirt under the outer shirt, which I would reccommend. About the only draw-back that I've found is that the straps could stand to be a little wider, which would increase the comfort of the rig. Either wider straps or just make the straps wider at the points where they cross your shoulders.
I'm sure that everyone has their favorite holster, but if you don't have a shoulder holster, and would like to try one out, you can't beat the Galco for it's concealment and comfort, and the price won't won't have you running to the bank either.