PvP: Hamstring is probably the only ability that the Warrior must always must use no matter what, or risk a shameful death by a cloth-wearer's fun little kite. Improved Hamstring gives a rather large chance to give the Warrior five whole seconds to smash his/her victim's face. As a bonus, it has a great psychological effect (namely, panic) on target players. Suddenly not being able to move forces them to use cooldowns they wouldn't have used otherwise.
Bottom line: PvPers, if you're going deep into Arms to get Mortal Strike, you better stop by to grab Improved Hamstring. A complete immobilizer (instead of a snare) is the best for a Warrior, who always needs to be close to do massive damage. It will turn the tide in most battles, pretty much guaranteeing you win 15% more of them (if you use it right). However, getting this isn't just an "I Win" talent; some degree of skill is needed to fully utilize this talent. Learning to use Hamstring at opportune moments helps. Mindlessly spamming Hamstring for this to proc is wasting Rage, and shooting yourself in the crotch.
PvE: You may pass it up and put your 3 points into something more worthwhile, like talents which increase your general DPS.
Patch 4.1.0 (26-Apr-2011): Now reduces the global cooldown on Hamstring by 0.25/0.5 seconds in addition to its current effects.