Been lurking here for a couple years, decided to finally join up. My first post, so be gentle!
The real problem with Henderson isn't whether he's crap or if he's the Second Coming- the issue is, what is his position/role in the team's midfield? There are 4 basic types of midfielders in the modern game:
Defensive Mid: either a destroyer (Mascherano) or volante (Lucas)
Holding/Playmaking Mid: Alonso, Javi Martinez, elderly Scholes
Creative Attacking Mid: a "between the lines" playermaker like Modric, Silva; or a trequartista like Kaka, Totti
Attacking Mid: direct ballsy type like Gerrard, or "flair" wide player like Ronaldo at United
So, what is Henderson's role?
He clearly isn't a defensive midfielder; he has some of the worse stats on the team for tackles, possession duels, possession duels won, ariel duels, and ariel duels won.
Is he an Alsonso type holding midfielder? Not really. He has a good pass % rate, but a pedestrian pass completion % for passes over 15 meters, and below average pass completion % for forward passes over 15 meters.
So, is Henderson an attacking midfielder? He's had around 35 PL games per season for the last three seasons, and has a very, um, pedestrian goal scoring record each season. But as we all know, there's more to being an attacking player than just goals and assists. So...
Is he a creative player? This season, not really, he's averaging less than one "chance created" per game (the supposed strength of his game).
But even beyond that, there is another measure of whether a player is a "danger" player- Fouls Suffered. How often a player is fouled is often a good benchmark for whether they are creating dangerous situations- the player with the most Fouls Suffered in the PL this season is none other than our own Luis Suarez (over 100 Fouls Suffered). Henderson has less than half as many Fouls Suffered as... Stewart Downing, and about as many Fouls Suffered this season as... Lucas.
All in all, my humble opinion is that Henderson is hardly "crap," but is unlikely to develop into a elite Champions League level of player, but could serve a useful role as a higher end, "utility" squad player. That said, I hope I'm wrong!