Shower pumps... Any good ?

Definitely - only if they are fitted correctly and sized to the system accordingly. They will improve the water flow by a factor of 4 when comparing to gravity feed only. Buy cheap buy twice is the rule to follow regarding pumps.
Fitted today in Solihull - We supplied and fitted this quality single impeller shower pump to this hot water cylinder. Note we used full 3/4" pipe throughout and soldered joints to effect a permanent leak free fitting along with full bore shut off valves for quick access to pump when needed for any servicing. We also included a new fused spur installed by our sparky. This pump increased the pressure to all 3 mixer showers throughout the property much to the delight of the tenants ! Full details of options and prices here >