I voted no - and that applies to both of my OS sims. I work and spend time on them with my viewer out at 350+ draw distance or on Hyper - and have no problems at all.
But then we made the decision at the point of starting the project that all textures would be optimised, nothing bigger than 512x512, and all scripts would be experimented with to get them as clean and efficient as possible.
We therefore ended up with around 2000 prims on each and a number of scripts including the usual (running water, poseballs, waves etc) but also a lot of complex scripts for animated sculptie creatures.
I've had up to 25 people on one of the sims with little or no noticeable performance deterioration. This number of simultaneous avatars was a very rare event, but a good test to see whether our efforts at optimisation paid off.