If the 'slim-jim' fit's your needs it should do as well as any other antenna of the same type. 'Gain'? Not really, it's basically a 1/2 wave antenna with an impedance matching system. It should provide more 'gain' than a typical 1/4 wave antenna, but nothing really exceptional. It's primary benefit is that it's portable, can be 'wadded' up into a small space, and can usually be hung just about anywhere. One draw back to them is that they won't usually stand up all by themselves, too flexible. If that satisfies your requirements for an antenna, have at it!
- 'Doc