I ran an undertank filter for a couple of years on my 748. The main reason being that the stock tank/airbox seal was terrible and the filter guaranteed no dirt down my throttles.
Earlier this year I fitted MPX038 filters in the airtubes and the difference was significant. Throttle response was improved and the bike generally felt more "lively".
I've now got a later seal to fit to the airbox to seal to the tank better and i'll be happy
IMO, i'd never go back to the undertank filters. The MPX038's are cheap enough to justify buying the later seal if required and still be quids in.
The Pipercross filters are also good enough to allow removal of the mesh in front of the airtubes to allow more flow and not disintegrate like the OE items.
[Edited on 7-11-2005 by nelly]