I use a combination depending on what I'm doing. I shoot only in RAW so this is the workflow that works best for me.
1) Browsing a few test shots - I'll use Bridge (comes with most(?) Adobe CS3+ software bundles) and simply preview images quickly in the file browser and take a look at a few just for a peek. I have Bridge open Camera Raw when I double-click on an image so it's basically a stand-alone solution when used this way.
2) Serious photo editing - if I have more than a couple of dozen shots, I'll import the images and load them directly into Aperture. It works basically just like Lightroom and allows quick adjustments to multiple photos, direct RAW editing, and changes are saved with the DNG file. I like Aperture because the interface is integrated into a single window unlike Lightroom that spreads it between Library and Develop but I actually prefer the print options in Lightroom.
3) Output to web pages, print, contact sheets - I'll do most of this directly in Aperture as well. Lightroom does the same types of functions and they probably work better for some people than Aperture (obviously Aperture isn't available on the PC).
If you're serious about photography and have tons of photos, you should really read
Real World Camera RAW by Bruce Fraser and Jeff Schewe. It is one of the best texts for learning how to integrate a practical process for a RAW workflow. I learned a ton about Bridge and Camera RAW that I still use to make my life easier.