I can remember lusting after a 540c when they were new, but all those zeros in the price kept me from buying, even though by then I was a working man. In retrospect, however, I now appreciate the 68030 PowerBooks more than the 040s for their energy saving features. Any 68LC040-based PowerBook was obviously faster, but the 040 chip had much more primitive power savings features than the 030, and the combination of the two (speed and lack of granular power management) sucks down so much more power in an 040 than an 030 (with energy saving features properly set) that the difference is startling.
With a RAM disk, Write Now or Nisus Compact, a minimal system installation, decent RAM, and a good battery, one of these 030 PowerBooks can be kept going for a working day (allowing for periodic, short system sleeps when idle, and esp. with the newer NiMh cells available). Then you pop in a spare battery and go just as long again. The 040 'books, though much more powerful, could not match that specific kind of performance, which was so suitable for lengthy spells of word crunching and the like (which is mostly how they were used back then). 3 hrs or so was tops for an 040, especially with later System software that demanded running it from the HD, so that in that sense, you got half the performance.
My absolute favourite, accordingly, is the 270c. Fabulous screen, compact, interesting technologically, lots of RAM expansion available, and running everything on a 33MHz 68030 with 68882: the 270c is 68k PowerBook bliss. Mine can run Nisus Writer 5 from a RAM disk like greased lightning, and that's with the processor set at half speed.