I have recently started using Lightroom after several years of using CS2 (with the Noiseware plugin).
I recently decided to move to raw and I had thought that I would just use Lightroom (version 3 came free with my camera) to load raw images and then transfer them across to CS2, with which I am very comfortable. However I couldn't resist having a little play with Lightroom for processing and before I knew it I had ordered an upgrade to version 4 and bought a book about it.
Not surprisingly there are things I can't do in Lightroom, but I am astonished at what I can do with it, and how easily, and with what good (to my eye) results. I don't have many raw images to play with yet, and the opportunities for getting more of my favourite invertebrate and flower piccies are a bit limited at the moment. So, in order to get some substantial practice with Lightroom I am currently going through a day's (750+) JPEG images from my backlog, from a shoot at a nature reserve in May. I am about a third of the way through and have picked and processed about 30 images (processed for screen viewing - haven't explored processing for printing yet). I have only been tempted to go into CS2 once, to do some cloning, but it turned out that I could do it in Lightroom after all. So thus far everything, including noise reduction, has been done in Lightroom, and a good proportion of the images are ISO 800, which is as high as I could comfortably go with my G3 when using CS2 and the Noiseware plugin. (Culling and image selection is a significant operation for these fairly large image sets. For that I assumed I would continue to use Faststone. Here too, I have not left Lightroom yet.)
I'm not suggesting Lightroom is a substitute for Photoshop or a similarly high-functionality application. There are obviously plenty of things that you simply can't achieve in Lightroom, for example perspective correction (which doesn't matter to me as it happens) or more general warping (which does).
And there are some other areas which have been implemented in Lightroom but seem a bit weak to me - cloning/healing is one (cloning is crude - complicated to explain - and healing tends to leave faint but visible outlines around the "healed" area). Highlights/shadows is another weak area IMO; you only have a couple of stops of latitude. I for one need more, and for difficult cases I will have to go over to Photoshop, where even CS2 has Shadows and Highlights functionality which not only allows more latitude, it also allows much more subtle control with its additional Tonal Width and Radius parameters.
However, there are some other things that I had assumed needed some Photoshop functionality that Lightroom doesn't have, but which it turns out can be achieved in Lightroom by other means.
For example, in CS2 I would use a soft light layer merge-down to enrich colours. Gentle use of the Vibrance slider in Lightroom seems to achieve what I need, more easily and quicker.
Or selective noise reduction. I had assumed this needed either layers and masks or, what I used, CS2 selection facilities coupled with native CS2 or plugin noise reduction. In Lightroom 4 you can define areas for selective noise reduction (and loads of other functions) using the Adjustment Brush, which has a very good auto mask facility. And where the auto mask allows overspill, you can simply hold down the Alt key and use the brush (still in auto mask mode) to reverse the operation. It is similar to painting on a mask in Photoshop. And having defined the area you can then decide how much Luminance and Colour noise reduction to apply, and the detail protection to apply in each case. It doesn't have the huge range of options the (for example) Noiseware plugin has, but thus far I am finding it sufficient for my purposes. (And btw if it isn't quite right you can come back to it later and alter the area covered and/or the parameters, independently from any other operations - i.e. you don't have to go back through an undo stack undoing other things that you don't want to change and do again. It is a similar effect to what you can do with adjustment layers in Photoshop etc).
And in some cases Lightroom combines two fairly straightforward facilities to provide a combination of power and ease of use that I have not seen before (but I am several generations behind with Photoshop, and so perhaps this is quite normal these days). For example, one of my first steps in Photoshop would be to look at the histogram and close up any gaps at the top or bottom. That is easy enough, but deciding how much further to pull in either the white point or black point is more tricky - you need to know what is clipping and I would sometimes use the separate Threshold function to find out the distribution of highlights so as to get a better idea of what I could get away with. In Lightroom 4 the Whites and Blacks slider seems to be very similar to moving the white and black points in Levels in Photoshop. But, if you hold down the Alt key while using the sliders you get a Threshold view, which shows separate channel clipping as well as full 3-channel clipping, see below. So simple, but so very effective, usable and quick.
There is something similar with sharpening. If you hold down the Alt key while using the Amount slider the (part of) the image you are looking at goes greyscale, which makes it much easier to see the impact of sharpening, including the adverse effect on noise. If you hold down the Alt key while using the Radius or Detail slider you get to see, in greyscale, which areas in the picture are going to be impacted by the sharpening, the areas changing as you use the sliders. The Masking slider stops sharpening being applied to areas with no or weak edges, and if you hold down the Alt key while using the Masking slider you get to see in greyscale where the sharpening will and won't be having an effect, see below. Again, simple, but very effective in function and usability.