The Diamond RH77CA is a very good antenna for UHF and VHF. Public Service in both bands are double what your stock duckie will recieve. But, as mentioned, it isn't much for 800 mhz. I've found it does decent on 800 mhz however, on my Pro-528. From Irvine, KY, I was able to recieve Lexington, KY's EDACS system, and thats about 25 miles away as the crow flies. I was also able to pull in Richmond, KY's Motorola Type II system, which is about 15 miles away.
With the stock duckie, Lexington's VHF Police did come in, but it was scratchy, and the audio was low. I threw on the RH77CA, and the hash and trash was gone. Audio seemed to improve a small amount as well.
For 800 mhz trunking, and even 800 mhz conventional, I'll have to give my award to the RadioShack 800 though. It's a decent antenna for VHF and UHF, but it really shines on 800 mhz.
The only downfall is, the RS 800 is almost deaf on VHF aircraft. The RH77CA is a proven air-band antenna..