As an owner of an NEX-6 and two of the more expensive lenses currently available for it (the Sony 18-200mm zoom and the Sony Zeiss 24mm F1.8), and with the warning that I may be doing nothing more than reinforcing my own biases, I would say that, if they meet your needs, you shouldn't hesitate to purchase E-mount lenses. Why?

First, the mirrorless camera industry in general and Sony's NEX line specifically seem to be doing quite well. My evidence is anecdotal, but clearly there's a great deal of market momentum in the mirrorless segment, and Sony has received quite good reviews for their NEX cameras. Four of Snapsort's top five mirrorless cameras are NEX models, and the only one that isn't, the Leica M 240, costs approximately seven times that of the most expensive NEX model.

Second, the E-mount system has excellent choices for a variety of purposes now. Sony themselves offers 13 different lenses plus two converters in the E-mount format, with lens list prices from $250 up to $1,200, and with focal lengths from 10mm to 210mm. (You can find the Wikipedia article with a good overview of the lenses here.)

Third, the E-mount system has attracted third party support. Tamron, Sigma, and Zeiss all make E-mount lenses with autofocus and full electronic control. I personally tend towards fewer better lenses, and so my interest is mostly in the Zeiss lenses. The image samples from their new 12mm/F2.8 and 32mm/F1.8 lenses have been stunning, at least to my eyes. And Zeiss has more lenses forthcoming, with an a 50mm/F2.8 macro lens due later this year.

There are plenty of lenses you won't be able to find that are specifically designed for E-mount. If you want an 800mm telephoto lens, or a tilt-shift lens, or a fisheye lens with a full-frame-equivalent aperture of 8mm, and you want them tuned for E-mount, with full autofocus and electronic control of the aperture, then you need to keep looking. But these are fairly specialized requirements; my hunch is that if you had them, you'd already know the answers and so wouldn't be looking at NEX -- you'd be focused on full-frame formats, probably Canon or Nikon.

Ultimately, a question you should ask yourself is this: given the E-mount lenses that are available now or that have been definitively announced for near-future availability, could you purchase a camera-plus-lenses set that would give you enough years of usage to make the original investment reasonable on a per-photo basis? If so, then dive in and have fun!