Rumors (and blog posts) are flying that some Whovian regions are using spiffy new mesh builds, with at least one sim going all in to completely revamp. As I've written here and here, I think completely redesigning a region with mesh builds is premature.
For one thing, more SL users are still using third-party viewers that do not yet have the ability to see mesh builds in all their glory — stability issues with Viewer 2 and Viewer 3, and strong dislike for the crappy UI thereon, necessitate sticking with Viewer 1-based viewers. This means that most SL users will not be able to see all those pretty mesh creations.
Another problem is in the cost of mesh imports, not only in terms of money but in prim-count. The more complex the import, the more it costs to upload, and the larger and more complex the build, the higher the prim count. While this may be fine for limited mesh importation in full-prim regions by people with money to burn, most people will find it cost-prohibitive. What good is rebuilding a region completely when prims are limited and too many people won't be able to appreciate the work put in?
Does this mean that mesh is too impractical? Initially, yes. But things won't be so for long. Third party viewers such as Cool VL Viewer, Singularity, and Phoenix have either already developed mesh-viewing capability or are on their way to bringing it into their systems. The biggest problem, then, will be the cost prohibition. Wiser land-owners will want to keep mesh imports to a minimum to help keep prim-count and lag down, as well as save money. It will probably take a massive user petition to Linden Lab to bring down the monetary and prim costs for mesh, too.
For a more positive outlook on mesh, here's a review worth reading.