Like you, Mike, I use PS mostly for photos. When I'm working on large numbers of images for an event, my first (and possibly only) pass through them will be in Lightroom, not PS. This has tremendously sped up my work flow for several reasons. First, it allows one to group the images that need different raw conversions into groups (eg, outdoors mid-day front lit, outdoors midday back lit, dusk with fill flash, dusk backlit without fill flash, etc.) and then, with a single mouse click, apply different raw conversion presets (identical to ACR presets) to each subgroup in the set.
The second way LR has sped up my photo processing is at the output stage. It tremendously reduces the labor involved in generating, say, from a set of a few hundred images, a few web-sized jpgs, a different bunch of full rez JPGs, print out a few others, and then export a select few as PSD files for further work.
Unless you are doing things like compositing, skin retouching, slenderizing / body-reshaping, text efx, etc. that can't be (easily) done in LR and specifically require PS, you may never need to use PS.
However, if you frequently do such things, there are some nice features in the more recent versions of PS that are *very* nice. Unfortunately, I forget exactly at what point these features were introduced, but I think that features such as the various content aware tools, puppet warping and improved liquify filters, anti-shake filter, the new ACR adjustment layer, improved text tools were all introduced after CS5.
If you are considering upgrading PS, but haven't yet taken the time to familiarize yourself with LR, I would do that first. Allow enough time as the basic paradigm is a bit different than PS, so there is a bit of a learning curve, but, IMHO, it's well worth it.
BTW, while one has to "import" files into LR to work on them, doing so doesn't modify your original files or directory structure in any way. Also, one does not have to use the image database management tools built into LR if you don't want to (... I don't). OTOH, if you don't already have a good system for image keywording, image search and retrieval, etc., LR is quite competent in this area as well.
Just my $0.02,
Tom