Here's my impression: it entirely depends on your riding style and conditions. On my TT bike, I have no interest in running 11-speed. My 10-speed 11-23 cassette can pretty much handle any situation I throw at it. However, on my road bike, I can tell you that the extra cog is very helpful.
Slowtwitch recently had an article on 11-speed. The crux of it is that the added cog added more versatility to your cassettes.http://www.slowtwitch.com/Tech/Cassette_Evolution_4171.html
I can tell you that this is true. Personally, I used to run a 12-25 cassette for flat riding, and an 11-28 for hillier rides. The 12-25 gave me a narrow range of gears with minimal spacing, so no big cadence drop-offs, whereas the 11-28 had huge gaps, but I used it solely for climbing. When I went from 10-speed to 11-speed, particularly when using the 11-28 cassette I have for climbing, the gaps are much smaller, the cassette acts like an 11-25 cassette, with a 28T gear added to the end. I get to have my cake (relatively narrow gearing in the lower part of the cassette) and eat it, too (a 28T for serious hills). The only drawback in my case is the weight penalty.
As for the wheels, many are upgradeable for a small cost. Reynolds sells new freehubs for less than $100; same goes for DT Swiss (I changed over the DT240 freehub on my TT bike's wheels to the 11-speed version, in case I have to use those wheels on my road bike). Of course, if you want to start the upgrade process on wheels that can't be upgraded to 11-speed Shimano/SRAM, you can always use a Campag freehub/cassette combo, which has been discussed on other threads. You can also run the 11-speed cassette minus one of the gears, as has also been discussed.