Is there any documented release from Microsoft stating there will be a "next generation" RT?
Also, I'm very curious as to what those who say it's underpowered are doing with their RTs. Are they looking at specs? Or basing comments on actual experience with actual usage?
I own both a Surface Pro (used for work) and Surface RT (consumption, left in the living room for everyone to use).
The Surface RT is good, it performs 'well enough' for most tasks and you can count on further optimizations from Microsoft in the way of making Windows RT work better on the Tegra-3 CPU. That said, you'll always face a risk of lag or slow loading time to time. The issue isn't that bad though, but I would prefer not to ever deal with it. Look at Windows Phone, it runs beautifully and lag is a very rare occurrence. The next-gen Surface RT is poised to use Snapdragon 600 or even 800, and you can be assured that it'll perform without a hitch. Alternatively, a Nokia tablet is also on the way, and you can be assured that there'll be some interesting exclusive apps and services on it.
$349.99 is a great deal, but if I had that money now I would much rather hold it and only worry about the other $150 in two months.
That said, if you're really looking to get one, and will use it to test Windows 8.1 as well as Microsoft's eco-system (SmartGlass, Xbox, Remote Desktop, etc), then yeah, it's worth it in that regard. You'll have a lot of fun with it for a few months, but be warned that a much better set of products are on the horizon.