Marcin, I don't say that Gammu+ is generally bad. Simply there is not enough man power for Gammu and you will only trash your effort by doing another project. Most of improvements in Gammu+ can be also implemented in Gammu and you will not loose support for lots of phones.
I simply do not think we're powerfull enough to make revolution in Gammu sources which you call Gammu+.
I haven't yet deeply checked it's sources, but I'm affraid of it a bit from few parts I already saw.