The shell is available for free in the OVI store and then if you'd like to keep it you need to upgrade to the paid version. For my NOKIA N8 I went straight for the paid version of the product because I had a feeling I would like it and so far I am pretty satisfied. Overall the GUI looks more modern (this of course is a matter of personal opinion) and I really do find utility in some of the new widgets that SPB brings to the table. There are a few minor bugs in the shell and I'll soon do a write-up outlining them. In the mean time I've been methodically reporting these bugs to SPB. Also, with a single button it is very easy to toggle back and forth between the SPB Mobile Shell and the stock Symbian 3 GUI so you don't have to be worried about getting "stuck" in the shell.
You might be wondering how much RAM this shells consumes and I was as well. So I ran a before and after diagnostic and it seems that while the SPB shell is operating it consumes about 18MB of RAM. That might seem like quite a lot but in my time with the shell I have not had many out of memory issues on my N8. I have had out of memory issues on a few occasions but those were very extreme cases with multiple heavy apps open along with multiple browsers.
In conclusion, SPB Mobile Shell is essentially a fresh way of presenting/accessing/navigating the same old information and apps that you're used to in Symbian 3. So if you're happy with the way things look and feel in Symbian 3 then don't bother with SPB Mobile Shell. However, I would recommend for people to try the free version and see how it suits them.