i'm not a mac user myself (never have been) but from what i gather, macs are much more stable than PC's running windows, the only thing i ever hear about crashing on a mac involves microsoft products (like office). Also there far more secure, as not so many people have them then virus writers (you know the 14 year old kids!) don't write viruses for them. In fact i've heard of competitions where nobody could hack into a server runnng mac os (or something like that, i forget). Also there not actually slower. You have to realise that megahertz and gigahertz is purely a term coined by intel to describe how fast there machines are (the frequency of the chip), however thats not the only thing that makes a computer (or mac) fast. Theres other things to consider, which i can't really be bothered to explain unless you really want me to. So there lastest G5 or whatever its called could be comparable in speed to the lastest intel chips.
True some of the lastest ones are expensive, but then you try buying one of the lastest intel computers and you'll probably see that there about the same price, and with performance that is quite similar too. It depends what you would want it for, there used a lot in graphics design etc.