I have been a dreamhost customer for >3 years now. I have had a .Mac account since 2001.
.Mac email is good and reliable. It is never down.
Dreamhost email had some problems during the summer months of '06 but these reliability issues have been taken care of and things run smoothly.
I have 6 or 7 more email addresses through dreamhost, all related to different sites that I manage. All of these addresses are set up through Apple's Mail.app.
I find the the configuration options of dreamhost to be essential in some situations. With options that .Mac can't offer.
By using a .mac account as my main contact email for dreamhost support I can ensure that my emails always com in a timely manner and that even if some critical problem were to occur then they would always have an emergency address on file.
I find that .Mac storage is quite sufficient for it's purpose while the bulk of my mail comes into dreamhost accounts which can be configured as virtual bottemless pits!
Since you already pay for dreamhost hosting and get all the email addresses that you could want then I suggest making your primary email with dreamhost which means a professional @yourdomain.com address. Keep your .Mac account around for backup... I know gmail or something could do this equally well but... .Mac syncing and other services are well worth the $$ you spend there so... that's my $0.02.