I installed the Logic Express demo, and, realising that most of my plug-ins (even the ones for OSX) are VSTis (which, of course, Apple's software doesn't support), forked out for a copy of FXpansion's VST-AudioUnit adaptor.
The good news: All the plug-ins seem to wrap nicely into AudioUnits, which Rax and GarageBand can handle. Virtual Guitarist works nicely.
The bad news: Logic is somewhat more picky, and refuses to run any plug-ins that don't pass a battery of strict tests it performs on any new plug-ins it finds. As it happens, this includes most of the Computer Music cover disc plug-ins and (most annoyingly) Waldorf Attack (an analogue percussion synth that's very good for making all sorts of sweeps, burbles, warbles and glitchy noises) and Native Instruments FM7 (both in the original Audio Unit format and the converted VSTi). Fortunately, its refusal can be overridden, but it still complains vociferously, and puts them in an "INCOMPATIBLE" category.
Can anybody recommend another reasonably inexpensive audio/MIDI sequencer for OSX to look at? I'm reluctant to go with Cubase SX/SL, as that, in turn, doesn't acknowledge the existence of Audio Units, and unlike for AU-based software, there does not exist an adaptor to remedy this.