Depends what you're looking to do with it. Here's the features post from the ModDB page:
Well when it comes to features SoHL has a lot: -weather effects -lightning -reflections -physics -supports advanced rendering (code it yourself) -supports bump mapping (code it yourself)
First off is the well know aurora particle system that has been implemented in SoHL, with this you can multiple sprites as particles to make rain, snow, fog, and clouds. The Locus system allows you to link events with entities and this allows for more advanced calc_entities. SoHL also added many new special effects in its fgd like: env_shockwave, env_quakefx, env_decal, env_warpball, env_rain, env_beam, env_laser, env_sprite, env_fade, env_dlight, and env_elight. Along with special effects SoHL has drastically improved triggers and scripted sequences.
My personal favourite feature is being able to change NPCs alliances and how they act, as well as their model and skin. So you could have a three way battle between red, blue and green grunts for example, without any coding.