My ASA 5510 firewall arrived, and it's lovely! It's faster than the Pix 515E; it has more memory, a faster processor, and gigabit interfaces. I swapped it for the 515E today, and it went in very easily. I just copied the configuration from the 515E almost word for word, and it's now humming away happily.
But to me, it's just a more advanced Pix.