Everything you ever needed to know about a Hilltop map is here..
http://www.ukgser.com/forums/showthread.php/348990-Hilltop-Tune?highlight=hilltop.
To sum it up:
Hilltop...most people are very happy with the extra grunt provided, does away with the flat spot and the concensus is money well spent. It doesn't affect the warranty as it can't be detected by the BMW boffins. Some say the bhp and torque gains are pure fabrication but most people don't care because the bike feels loads better.
Power Commander......same results but with the benefit that you can alter the mapping to suit your own personal style. Some say the PC unit can be affected by weather but most say it's money well spent. You have to remove when your bke is dealer serviced because the boffins don't like the bike being tampered with but it only takes 5 mins. Similar investment as the Hilltop but you can get some back when you sell it.
You pays your money and takes your choice to reach the same conclusion.
Lets not have another marathon thread ............please