Today I received USB scope DSO-2090. I bought it on EBay instead of my old analog scope. I must tell - Great thing. User friendly software interface. Bandwidth declared as 40 MHz - it seems to be true. I don't need greater. It works in real time despite of USB connection bandwith limit. You can auto set up all parameters (voltage and time) or you can set up them manually. You can see FFT spectrum as well.
Here is a box
And here are things inside the box. Scope, CD, short instruction, USB cable and 2 probes
And here I tried to reach scope limit. You can see signal around 3-4 MHz
You don't see squares because of high bandwith. With lower bandwith signal is pretty stright. All controls are like in analog scope. And you can measure everything - frequensy, rising time, duty cycle etc.