While they might not have a great radius they do a ton of damage, especially if you have sabotage speced for tech vulnerability. Not a bad power at all, doesn't deserve to be at the bottom of the list.
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I think 6 is fair. Their crowd control is awful, but they hit like a tiny bomb against armored targets. I think it takes four to take out an Atlas on gold. My biggest problem with them is that they're essentially a single target power, but they're not spammable enough for the role because they're grenades.
The small radius and delay make them more situational but sticky grenades are the ultimate boss killer. You can tag 3 grenades on a target without breaking cloak and have them detonate with the cloak damage bonus. Makes short work of fat targets like ravages, brutes and atlases or taking out spawn points. 8/10.
Serviceable enough, but I always feel like, by picking one up, I'm depriving my team of a more useful grenade. Stickies can hurt, especially on bosses, but I'd always rather my QMI teammate get another Arc Grenade to toss, the Demolisher have another tech combo, the Krogan get another Inferno, or Drell Adept another Biotic explosion, for instance. Virtually any other class that can pick up grenades can make better use of it than just a Sticky.