In regard to happiness, I cannot speak to a individual customers like or dislike towards DFP. However, it does have a rating of 4.5/5 in our tag marketplace.
We can speak to a few alternatives too.
This add from Jan 2014 speaks of a few. http://monetizepros.com/blog/2014/6-programs-websites-can-use-to-serve-ads/
This LinkedIn post has several comments you may find useful. While it is 2+ years old it can still give some insight into possibilities. http://www.linkedin.com/groups/Can-anybody-recommend-alternative-OpenX-154454.S.101322170.
I haven't heard of this site before but doesn't mean it cannot be trusted. http://alternativeto.net/software/google-ad-manager/
If you log in to Tealium, go to the Tags tab, click the green "Add Tag" icon, and go to the "Display Ad" tag you'll see all the Ad vendors that Tealium currently supports. This can give you additional ideas, and ensure you choose a Tealium supported vendor which will ease implementation.
Looking at internal reports, the top 10 Ad tags in use by our clients are respectively as follows:
Poke around and let us know if you have any further questions.