Hi David,
First up, I'm the author of StarTools, so don't believe anything I'm about to say.
Seriously though, if you're into planetary imaging and already have PhotoShop you're well on your way in the software department.
Paul Haese, one of my favorite planetary imagers here on IIS, uses PS for his work with great effect. One thing you'll probably want to invest in is a deconvolution plug-in (for example
AstraImage) and perhaps something like
GradientXterminator. Registax will already give you a basic wavelet sharpener, which a lot of people seem to like.
While I realize you already have PS, don't discount the GIMP (free) for planetary processing if you're just starting out. While it only processes in 8-bit, there are some very good free plug-ins available (including wavelet sharpeners and decon). At the very least they're a great way of familiarizing yourself with the workings of the various techniques that are at your disposal as an imager.
So now to answer your question. Yes I use StarTools for all my planetary post-processing (it's only natural I 'eat my own dog food'

) and deem it very, very capable.
Things you'll appreciate about ST will be the price (AstraImage + GradXT alone will set you back $100, while ST does the same better & faster for $60) and little things like near-instant wavelet sharpening (instead of RegiStax' rather slow implementation).
Depending on your gear and equipment, there are some other neat little tricks you'll appreciate such as the real-time luminance mixer and the Lens module (long story short they're invaluable if you don't have any UV/IR filter or your barlow is achromatic).
Lastly ST was created for astro imaging, so getting optimal results is *very* quick compared to a generic package like PS.
Here is an example of the fruits of a quick 2 minute ST session, fixing up a (compressed) image of Saturn that was taken during very bad seeing conditions.
There's a free demo on the ST site, so it may be worth giving that a try. By all means try ST with the images of other plug-ins' examples, tutorials or demo images. It's what I do during development to ensure ST produces equal or better results than what's out there.
Finally, let me know if you have any questions/problems and/or feature requests.
Cheers,