I don't know if it's specifcally worth it, but I have just started making sandwiches myself. They aren't exactly drop-ins though. I'm using 16mm cree star, that is thermal expoied to the top of the body, with wires running from the sandwich below it. I also use a Terralux tailswitch.
I'll put up some pictures eventually, but it worked out pretty well.
It's still a nice size, and if you have some mini-mags laying around, a good mod. There does seem to be a lack of 2AA drivers that can really drive a 1A. The BB1000 and Nexgen don't really seem to be able to do that.
My next one will be on a BB750 (Used a Madmax+ last time). Definitely brighter than the Terralux drop-in (which is quite decent), and the heat seems to be going into the body nicely from the epoxied star.
You don't get the focusing capability anymore, since I'm using the head to hold the reflector down with pressure, but I never thought the focusing was worth a damn to begin with, and don't use it with the Terralux drop-in either. You don't get focusing with any of the P60 type lights either.
I think they have their place.. I do think that they are worth doing it as a mod, but not buying a drop-in (too expensive). I prefer NiMH over Li-ION due to dangers associated with the latter, so I tend to stay away from the P60 crowd, though they are nice. Honestly a 2AA (3v/2.4v) driver running at 1A should be completely doable, but there just isn't anything out there that really runs effectively in that range. In fact there was a feeler/thread a few weeks back about the interest in getting a driver built for this express purpose.