My experience with preamps has always been disappointing with two notable exceptions:
1) old, low sensitivity radio. The preamp did its job nicely here, making the combination 'acceptable' - not brilliant though.
2) modern radio (two way repeater actually) with excellent filtering, quite good sensitivity. Placed a TUNED preamp on the receiver, before the combiner, and achieved approximately 5 dB performance improvement.
In my experience, preamps are definitely not a good thing to have if you're anywhere near the cities with lots of strong signals around - except if you are using a TUNED preamp, with a TUNED (ie single band) radio with good filtering.
Broadband 'scanning' type preamps are going to be of benefit if you're out on the fringes away from any nearby FM, TV, paging etc transmitters, and you have a radio thats low on sensitivity to begin with. A very sensitive radio (eg Yupiteru 7100 etc) probably wouldn't benefit at all from a wideband preamp in this situation, and a narrowband tuned preamp would likely only make a minor difference.
Also note that the higer the gain, the worse the results can be! For example, my tuned preamp above only had 10dB of signal gain - and it only produced 5dB actual improvement. That's normal. If I put a 20dB gain preamp on there, it worked far far worse - in fact worse than if the preamp was not there at all !!
Another thing to take into consideration is Noise Figure. The higher the NF of a preamp, the worse it will be. Broadband preamps are noisy - about 5-10dB NF usually. Narrowband tuned preamps are better, with NFs of 0.5 to 3dB depending on just how good they are.