I don't like them.  Let me say that straight away.

 

I have seen many stuff ups from CNAMES.  Most recently, someone used a  CNAME as their name server records for a domain.  It mostly worked - but some fussy DNS resolvers, such as the ones Bigpond use on their mail servers refused to resolve them and as such, they had time outs.  Took a little while to work this one out.

 

Other reasons they are bad:

 

Slower:  To lookup a CNAME, I have to make a request for the CNAME... and then a request for the A record of the returned query and then do my normal business (ie, send an email via SMPT).  A little more info here

 

Compatibility:  CNAMES are mostly client sided and as such; everyone seems to do it a little differently.  Due to this, there is inconsistency’s in how it works (some client software will work with CNAMES as MX records where as others will break and die).

 

Bad things can happen:  You can only use one CNAME to point to a host and not to another CNAME.  I've seen someone do a MX record of their domain point to their hosting company.  At a later date, the hosting company changed their mail server to be a CNAME to another server.  This broke things, of course.  You are not meant to use CNAMES to point to MX or NS records.  A little more info here

 

So saying all of that, CNAMES are bad. Stick with real records and just keep them up to date.  Don't be lazy and use CNAMES because it will bite you one day and you could spend some time thinking the issue is somewhere else (Like the Bipgond example at the top)