I've been using Lightroom 2.5 for awhile on my aging PC, mainly for organization and rudimentary post-processing. I use the presets a lot more than I'd adjust the levels and contrast individually.
My basic workflow is: Import to Lightroom->Selection and Processing->Export to JPEG->Upload to Flickr and/or Picasa
Now I have a much more powerful MacBookPro, I've been toying with the idea of either upgrading to Lightroom 3 or switching to Aperture. I am experimenting with both demos and found Aperture strangely unintuitive (probably because I'm used to LR), but I love its Flickr/Facebook/Email integration. [edit: ok, I'm an idiot, LR3 has it too now]