Ettercaps are a staple in the lower level games I run. They are tricky, use combined arms tactics, have the intelligence to withdrawn, complexity and function in three dimensions - something most people have difficulty with. I love 3-D environments, I grew up spending summers in whatever large body of water was nearby, I fly private aircraft and rock climb, I get functioning in 3-D. The average person only thinks in 2-D most of their waking life (read the section on the nebula).
So what's special about ettercaps?
Well, for one, they move in ways the PCs often cannot follow, meaning even if the PCs win the fight, they may not get any treasure. Ettercaps pretty much always have giant spiders around. They are smart enough to created complex traps. And they are straight-up bad guys, there's never any hesitation on whether to kill them or not.
What's the best use of the Ettercap?
I almost always give the PCs an opportunity to spot the webbing and nest of an ettercap, often with clues on the ground such as bare bones or empty insect husks. However, the PCs usually spot the spiders (who aren't hiding) and immediately underestimate the danger they are in. Which is usually about the time I spring the traps/web attacks. I try to have at least one snare to either hook a PC or push them towards more interesting traps. The more interesting traps are usually deadfalls underneath where the hooked PC is hanging or in the most obvious advance/retreat path, in case the PCs decide the fight isn't worth their time.
The poison isn't so impressive unless you can get it on a low dexterity (and poor save) PC. However, combined with the various spider types (no, I don't like for all spiders to have 1 type of poison, snakes having a different type, scorpions having yet a different type, I randomize what poison a creature has), and selecting traps to augment those poison types, you can have quite an entertaining combat.
Lest I forget to point this out, because of the Drow's love of spiders, I have also been known to put ettercaps near or in Drow areas to add to the sense of dread a party should feel going into Drow territory.