1. 5.51 works, and will work well right until a new Windows OS comes along (and it may even work after that!)
2. current mac's are toys. You really can't do much on them compared to a stupidly cheap AthlonXP 1800+ machine. On top of this, the reasonably fast dual macs are super noisy and generate a lot of heat.
3. They use PC133 SDRAM - laughably slow.
4. There's hardly any VST plugins for OSX right now, and you can't use them in Logic anyway. This wouldn't be so bad if there weren't even less AudioUnit plugins available.
5. OSX needs improvement. But I do acknowledge that it's a cool OS potentially and I really really really really wish they would port it to x86 platform.
6. There is no Pulsar/SFP for OSX!!!
7. Apple's approach is not future-proof. If you buy a PC, the hardware and software will still be usable on future computers. In Apple-land, this is not the case. This has constantly been proven in the past with switch to PowerPC, then the phasing out of floppy drives (nightmare for plugin customers), ADB ports (nightmare for anyone with a dongle), and now the OS.
Look at Wintel by comparison. I love the way a plugin written for win95 will still work perfectly in WindowsXP. Much as I hate Microsoft, I do admire this backward-compatibility approach from them.
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Having said all this, I will probably still move to Mac in 1 or 2 years when/if they finally get some vaguely decent processors out, with ability to use decent RAM. My love for Logic is too great... Samplitude looked like an option for a while but I couldn't believe you couldn't create an empty MIDI object and start putting notes in. Anyway even if they improved Samplitude a lot, I would still miss the Emagic plugins, which I use far more than any others.
Anyway, I'm happy to wait 1 or 2 years. The platform I use now works great and will continue to do so. I think this urge to go Mac NOW is just down to upgrade fever - you see a few cool new features on the marketing blurb and feel you NEED IT NOW.
You DON'T : you only think you do, and it would be very sensible to wait until Logic 7 / Samplitude 9


by the way, I didn't want to upset any Mac people with my statements - just pointing out the facts as they appear to me.
peace