I use Lightroom4 for 90% of my editing (I am semi-pro)... And for stuff like cropping, color corrections, exposure, contrast etc. it is a very powerful tool.... I have been finding it is very limited for other editing though. (I think of it as lightroom is my "darkroom" and handles all the same sort of stuff as a film darkroom... But any creative or retouch editing (like one might do a lot of for portraits etc.) is a bit beyond it's scope, or at the very least a bit complicated to manuver around for such things, and for that I tend to use Photoshop Elements or a similar all around editor)
I primarily use Lightroom 4 for doing minor edits, or for nature/landscape shots... However for most of my portraits I use Photoshop Elements or Serif Photo Plus (which is much like Adobe's Photoshop Elements, but a bit cheaper)... So if you do mostly nature type photography I'd say Lightroom is a great tool, if you do some portraiture as well, then you might look at Photoshop Elements (or Serif Photo Plus) which runs much the same for price but offers a wee bit more flexibility in the options.