I had a full custom map on my old S2R 1000 and I've had the Rexxer map for over three years now on my Evo. Comparing the two, I'd say the reflash was 95% as effective as the custom map.
The majority of the Evo's problems stem from the noise/emissions restrictions; remove those, (which the Rexxer reflash does), and you're over half way there. Another factor is the lean fuelling low down, (which even the DP Termi ECU still has - certainly for the homologated system). Again, the Rexxer reflash addresses this.
Yes, the reflash is 'off the shelf' and not bespoke but then you could say the same about any stock map - the factory don't dyno each individual machine, they just copy and flash the map from the original development bike.
Rexxer offer two maps; one for standard and one for slip-on + performance air filter. They will have developed the map using an actual bike on a dyno and you're just getting a copy of that map. As long as your bike doesn't differ dramatically from the Rexxer examples, the results should be pretty close to a custom map.
Considering the benefits against the cost, convenience and the ease of carrying out the change, I'd say it's the one mod you should definitely do.
Have a look at Chris.p's detailed report on his experience. It was after reading this that I decided to have my Evo done.
http://www.ukmonster.co.uk/monster/s...ad.php?t=50780
Note that the lumpy low speed running is not wholly down to emissions and fuelling - the lightweight flywheel also plays a part, but that's another story!
http://www.ukmonster.co.uk/monster/s...ad.php?t=54450