I don't really know how they managed it, but after the pretty excellent Opposing Force, Gearbox dropped the ball on this one in every way I can think of.
Confusing and shockingly bland level design, at the quality of a mediocre user modification, not a professional developer. Made even worse by the annoying backtracking, the escort mission, and the particularly dreary Xen chapter. You only access the most basic weapons, and fight the most basic enemies, and nothing very exciting happens at any point. Certainly not compared to HL1 or OpFor.
Extremely short, yet still outstays its welcome. Can only be recommended to the most dedicated Half-Life fans.