Pentatonic scales are just based on 5 notes whereas the typical major/minor scale would use 8 (8th being the start of the next octive). Of course you can 'go to town' on this theory stuff. The chromatic scale uses all 12 semi-tones etc. etc. If you lookup pentatonic on wikipedia you'll find even pentatonic has lots of different versions and meanings.
I learned the basic Pentatonic forms as you described and they are quite helpful, as they are pretty easy to memorise (hey, there is only 5 notes and you can play the scale with 2 fingers instead of 3 so why wouldn't you). I am sure there are a number of players who do quite well with them, so I would definitely give them a go and it does present a pretty simple way of finding your way around the fretboard
........but unfortunate for me they didn't take me very far and I ended up chucking the guitar in for a number of years because I didn't really improve.
Planetalk is not about pentatonics (but is about music generally and the guitar), and for me opened up many more possiblities for playing. It has given me a great way of viewing the fretboard, knowing how to find 'good' notes for any chord progression, and how they link to other 'good' notes along the way. The planetalk book will also give you a good plain english overview of music theory...I found this invaluable.
BTW ... I haven't done away with Pentatonic forms, major scales, or anything else I have learned, so I don't think Planetalk is a replacement for this stuff, but it is a great weapon to add to your armoury. I now tend to use my pre-planeltalk knowledge as backup material.
I know this reads like a testamonial and probably is. But I bought the book a year ago and haven't looked back.
Good Luck!