If you are considering spending $400 on a PIX 525 you might as well just
buy an ASA 5505 for $150 less.
The ASA is much more powerful, has a more mature/complete IPv6 implementation, and still receives updates from Cisco. The only major drawback is that the "10-User Bundle" only allows you to have 10 hosts behind the firewall talk to the outside world at any given time. But if you are using this for your home network or a lab that probably won't be an issue. But the ASA's IPv6 implementation does have a number of
important and annoying limitations that also apply to the PIX as well. (This might be good reason to go with something like Vyatta or pfSense.)
There are other restrictions on the "10-User Bundle" 5505 that don't apply to say, the "Unlimited User Bundle" or the "Security Plus Bundle" 5505. All of them are the exact same piece of hardware, the only difference between them is the activation key stored in flash that tells the OS what features to enable and disable. Cisco outlines this pretty well here:
http://www.cisco.com/en/US/products/ps6120/prod_models_comparison.html#~mid-rangeSince the only difference is a software activation key upgrading to a higher feature set later is pretty easy. The ASA is also guaranteed to already be running an IPv6 capable IOS image right out of the box so you won't have to worry about trying to find an IOS image for it on some shady torrent site.
Still, a PIX 515 for under $200 with the right amount of ram and the proper IOS image isn't a bad deal. Especially for a small LAN or lab where the higher capacity of the ASA 5505 just isn't needed.
