I think that these are a great idea for certain types of bikes. The obvious upside is not having to do chain maintenance. The downside is that internally geared hubs are less efficient than derailleur systems. I am a little curious if they can be adapted to any internally geared hub or if the hub has to be specially made to support the belt cog.

Not worth buying a new bike for, but if you were in the market for this type of bike anyway, it's an option worth considering.