Additionally, I built some logic into my spam filter where mail could be rejected (or the spam score increased) by the secondary if the primary was known to be up.
Another fine idea. For my own use case, the previously cited woes, fixable though they might be, served to make me more seriously weigh whether backup MXes were worth the bother, and judge whether a single-MX scheme could be practical and reliable as alternative. The answers turned out to be no and yes, respectively.
Yes individual requirements definitely vary. In my case though the primary and secondary MX's are just MX's - there are almost no mailboxes there. Most of the time these just forward directly to MS Exchange servers at completely different locations, having filtered the mail (spam and basic virus check). This way I don't have to rely on MS Exchange's crappy spam filtering, and I don't have to expose MS Exchange directly to the internet, and with multiple MX's at multiple sites an outage at one site doesn't affect the overall solution. The configuration is all in a replicated LDAP database so I only need to configure it in one location, and can add additional MX's as required (although I've never needed more than the two). James