I'm with Randall. I have had my Renzetti for over 5 years and have tied hundreds of flies and have never had a lick of trouble from it. If the o-ring ever does give out, you can replace it for less than $1 with a new o-ring from Lowes.
As far as the rotary feature is concerned, I rarely (never) use it to spin flies around and around, but when tying on curved shanked caddis hooks, I find it easier to dub, etc (without clipping the thread on the hook point) by rotating the fly 90 degrees toward me when wrapping the lower portion of the fly (as close to the bend as possible) and then sitting it back upright to complete the fly. Couldn't do that with a stationary vise. Its nice to have an option like that, especially if it doesn't cost any more...
"Even a fish wouldn't get into trouble if he kept his mouth shut."