hmmm...
at present I have three "sandboxes", and while one would expect a sandbox to be the same as any other sandbox, well, not what I've found.
So I do use Reaktor a lot to create all sorts of goodies... but truth be told, thus far maybe 1 in 10 is a keeper, and that's optimistic. I'm not ditching it, but neither am I focusing on it, and maybe that's my problem? Rapt attention has never been used to describe me<G>.
I also use pd - in many ways I would argue that this is the most fun one can have in front of a computer screen. My biggest problem is that I very often end up with something that has nothing to do with what I set out to accomplish. Frustrating some times, serendipitous some times... but I haven't ditched this yet either.
And I'm still a fan of Infinity from Soundquest, except for the part where it is getting a bit long in the tooth, probably because it did not fly off the shelves. Pity too, because in many ways it was a pioneer, and it remains quite cool, but I'm nervous that at some point it will simply stop working with the current OS.
Another really cool, and great sounding tool is (was?) Quantum from dbAudioware. In fact I'd say Quantum could give Max a run for it's money. I never did have the funds (or the justification maybe?) to purchase a license, but I spent many an hour with the demo, and it was great.
The new kid on the block is Bidule - from Plogue. I've tried the demo a couple times now, and the most recent pass was really quite remarkable. I just wish I had a better idea of what they are going to do about pricing... I don't want to invest in something that I won't be able to afford when it is finally released<G>!
And of course there is MAX... nuff said<G>!
I think Reaktor and pd make a very interesting, and powerful pair of tools that compliment each other in many ways. So while it isn't "cheap" in terms of time, I'd heartily recommend it!