I'm still using bits from Creative Suite 1 (Photoshop 8) through 5 (Photoshop 12) as when I don't need the extra functionality of the new version it's usually quicker to fire up the smaller & nippier old version.
Also some plug-ins I've invested in don't work on CS5, some don't work on CS4, etc... and as the company has either been bought out or closed down it's not likely they'll provide a 64-bit Snow Leopard CS5 version.
You can download a legal trial of anything you're interested in from the Adobe site (it is quite big), and then when/if you decide to buy it, simply buy from the cheapest source.
I usually wait a while for the prices to drop, or buy the old version at considerable discount once the new version is released and then find a cheap upgrade path.
Sometimes I can get a client to pay for it for me when I tell them "I NEED this new bit to do your bit of work. I can do it manually but it'll take another couple of days to do it" (which is usually true, and if I already have the software then I don't need to do this). If they say no, then I simply get a couple of days extra pay to do the process manually, if they say yes then I get less pay from that client but I get the latest software and I have some free days to work for someone else (i.e. free software PLUS extra days pay).