Solo ads, yes, as long as there's not too many. Multi-site solos - NO
The reason I say that is because those multi-site or "network" solos absolutely
kill CTRs at the mailers that have them. Some of the mailers I belong to started off
being very responsive until they added network solos, and then the CTRs plummeted to nearly non-existent. Some to the point I don't even use them anymore.
Oh sure, they're great for using to reach more people without having to join more mailers, but the members of those particular mailers suffer because of it.