Cheap 1MHz oscilloscope
The DSO Nano is a poor choice for a hobbyist's oscilloscope.
Capabilities
It's really only good for audio bandwidth signals - anything above 100kHz is going to be visible, but unmeasurable.  By most standards you want the sampling rate to be 10x the bandwidth of the signal in order to get close to measuring significant parameters of the signal (peaks, rise time, frequency, duty cycle, etc).  While you can see signals with as poor as a 5x sampling rate, you can't reliably measure such signals and expect to get good results.
Competition, New Oscilloscopes
Keep in mind that a very low end entry oscilloscope NEW is only about $400 - so if you can spend more then a 2 channel 50MHz Rigol would be a fine choice as a beginner:
http://www.dealextreme.com/details.dx/sku.30573
Note that the 50MHz rating is the fastest signal it can measure.  It measures at 1 billion samples per second for one channel, or 500 million samples per second for both channels, which means you'll be able to see signals much faster than the rated 50MHz.
Compared to the single channel, 1 million samples per second Nano this is worlds better.
Competition, Used Oscilloscopes
But even if that's too much, you can get a $50-$100 used oscilloscope on ebay that is significantly better than the nano.  For the same price you can easily find a 2 channel 20MHz scope (which can be used to measure the clock of a 40MHz micro, just not precisely) and there are many scopes in the 50-60MHz range for under $100.
What is it good for?
The nano is NOT an oscilloscope replacement.  The major reasons to get one are

You want one (it's open source, and could be fun to hack)
You need a portable device that can function as a simple oscilloscope for audio bandwidth applications
You are only interested in high-level overview of the signals you measure, and won't really be doing significant work with it (ie, beginning hobbyist) so don't want something bulky that you might only use a few times a year.

That being said, despite its severe limitations, I can see it as an additional tool for the workbench after you've invested in a good oscilloscope and other gear.  Keep in mind that many tasks, such as SPI, can be slowed down to within the range of the oscilloscope, and even though it's only good up to 100kHz analog signals, one could reasonably measure up to 500kHz digital signals (if they wrote software for the Nano that synchronized the ADC clock to the input signal).
Conclusion
The price to performance of the device is just so low that I can't see it being worth the cost except to tinkerers that might want to hack it.  Your money is better spent on a used oscilloscope.