I chipped my last car (petrol based Saab 93 Turbo) and any real world economy improvements are outweighed by the fact you are likely to spend more time with your foot down experiencing your new found power. Indeed, after chipping, stainless steel exhaust, Vectra VXR intercooler, air filter and a few other little mods to the engine set up my Saab was returning under 25mpg (from 30ish standard), which was just unacceptable with a 25 mile each way commute every day. Hence the switch to a diesel...
I think that you have to be careful due to a lack of out of the box units. The unit I used on the Saab was a BSR PPC, which was an out of the box set up through the OBD-II port uploading the 20 most used maps, with the ability to "read" the car and have BSR send you the maps most useful for the engine parameters you were recording (i.e. a Swedish car would get a map with a set up that deals with the denser air it has in its intake as opposed to a Dubai based vehicle for example)
Anything that's done on the fly is going to suffer problems unless you have a very good mapper, and ideally one who knows the engine very well. By doing it on the fly you get a map for that day's parameters, be it warm, cold, damp, etc, and everything else is an approximation relative to that initial map.