I bought my GP32 for Spectrum emulation and I would say Speccyal'K is the best there is at the moment. It is not as slick as ZX Advance on the GBA but it has two major advantages which are 1) You can have as many save states as you want and 2) the GP32 is able to give a much better screen resolution than the GBA so you don't have missing lines.
Watch out for fZX though, as from a recent interview, Rlyeh made it clear that he got into GP32 programming for Spectrum emulation (if you don't know, RLyeh is responsible for many of the finest emulators available on the GP32 - fSMS, FMSX and fGB in particular).
The GBA is a fine alternative to the GP32 for Spectrum stuff though as there is ZX Advance and some brilliant conversions of Elite, Lords of Midnight, and Cyclone.