IMO using an ERGO is not a good option for a hip carry and frankly it's a pain in the rear to modify the straps for it, unless of course you do it once and leave it there. BUT if you are in a pinch and it's the only carrier you have handy, it's nice to have the option to do the hip carry.
That said, I have never used the hip carry other than to demo it to others. I always use a ring sling, wrap or even my mei tai over the ergo for baby on my hip. The problem, like you said, is that the shoulder strap does not stay put and rides up to your neck . Not comfortable at all.
As a back or front carrier, however, ERGO rocks!
Zachary (Jan. 16, 05), Blossom (Sept. 14, 08)