I can only provide my own experience. The Soundtower products aren't perfect, and sometimes they do act up but I generally find they get the job done, and I'm better off with them than without. I typically run the standalone version for editing the instrument, with the audio going through Cubase. This is because the standalone versions have some functions the plugins do not with regard to morphing etc.
As far as support, I've sent a couple of emails and more than once gotten well-thought-out and well-written lengthy responses from Mark (I assume he is the primary developer/business owner). I do get the impression this is not his day job, and to some extent that sentiment is reflected in literally every third party editor for a hardware instruments I've ever used... and by that same token I can fully imagine that there are times when any small business owner gets so far behind on responding to every email that it just becomes overwhelming.
So, individual mileage may vary. Codeknobs editors are pretty good too -- I like that I can run a plugin that is full featured with regard to morphing/patch creation. They aren't perfect either though, and unfortunately there is currrent not an Codeknobs editor for the P5/P10. So in that regard, Soundtower is probably the best option for an editor.
Getting back to the original thread question.. I'm surprised nobody has mentioned Syntorial for learning sound design. It's not based on the P5, it uses a built-in soft synth to instruct but the principles of sound design are mostly the same on any subtractive synth. It tends to go on half-price sale around the holidays.