I`ve used Refco`s Startek. It is good for R22 and similiar only in a machine area. If you enter in cold room store where food or vegetables is stored it becomes crazy. And for R404 it doesnt worth 2 cents. It will detect just leaks that you can hear with your own ears.

Few weeks before we bought for test REFCO`s XP-1. Works just great. We`ve found a leak in 1400sqm supermarket. It was very small leak on evaporator and detector guide us from 50-60m distance. Point is to set it to max. sensitivity and walk to direction where it start to beep faster. When beeping is the fastest set down sens. and repeat procedure.
Bad thing about it is probe because it loose it`s sens after cca. 40 working hours. But cost of 1e/hour suits us comparing to cost of refrigerant, working hours in case of filling the system with dry nitrogen....

It is reliable, not so expensive (cca 300 euros) and easy to use. Worth every cent and lot more.

I`ve heard from my collegues that ZX-1 is great too. Maybe even better because it uses heated penthode as a sensor.