Awesome. Going down through the natural entrance is awesome -- descending the sheer switchbacks below the bat observing auditorium, and experiencing the gradual transition from a rocky to ornate cave along the way, there are many impressive views (like looking back up past a little waterfall towards the natural entrance) and formations to wind your way through (like a building-size rock that crashed down from the ceiling some time ago). Definitely enter through the natural entrance and not the elevator.
The bottom cave is huge -- it's laid out like a big cross, and a narrow path makes a big loop through it. There are additional levels above and below (no access), and various other passages, some of which are available by guided tour. There are other caves in the area too -- such as Spider Cave I think. But topside there's not much around (except Carlsbad) -- it's pretty desolate, but beautiful.