Another card so powerful it doesn't have a review. I'm writing one because I didn't think Legwork was that good initially, but experience has made me a believer.
You get to see more than half the corp's (default) maximum hand, often netting you Agendas if they're holding onto them. Otherwise, you get immediate information of their deck archetype and what they're possibly about to play next. It could be important assets that you can trash, dangerous ICE to prepare for, or combos you have to be careful of - anything that would let you make more informed decisions.
When compared with HQ Interface, you are only accessing 1 additional card. However, being low cost and click efficient, Legwork synergies better with the aggressive runner. Desperado and Gabriel Santiago: Consummate Professional's ability for example, would net you access to 3 different cards and 1 in a single action. Taking 2 more and 1 to install, HQ Interface pays off better after 2 or more accesses - efficient in the long run, but a much bigger hit to your tempo. You can always just play another Legwork after a few turns.
The question is, when to play this card?
Whenever HQ has 3 cards or more and you have no idea what most of them are (and obviously, when you can access HQ in the first place). The only thing you have to be careful on, like all multi-access cards, are ambushes. Take extra caution when using it against trap-heavy flatline decks.