I am not sure about how I eventually rule this feat, but a few things to consider:
Combat Expertise requires a Int of 13+. Not all characters have this. Bards, Fighters, Rogues and Wizards are the most likely to take this feat.
From these, the Fighter is probably more interested in "Power Attack, All The Time" then actually using combat expertise.
A Bard or a Rogue doesn't have a lot of feats and also has a medium BAB, so it comes at a great cost to him, and he will mostly become useless in melee combat. Many Rogues and Bards don't have a high Strenth, meaning that trip attempts will fail often. A rogue will probably prefer to fight and sneak attack his enemy, though against Undeads and Constructs he might really enjoy this feat. A Bard is usually better of casting a spell to buff his allies, hinder his foes or start a new bard song. Melee is also dangerous for him, since he doesn't have high HP.
A Wizard will usually avoid melee alltogether, so he would rarely use the feat.
Aside from that: Sometimes I find the usefulness of Combat Expertise itself quite limited - when engaging high level foes, their attack bonus is usually so high that Combat Expertise doesn't help that much, and I'd better concentrate on hitting and dealing the most damage.