I've tried all sorts of renderers with sketchup, but have found that none of them work as well and fast as Artlantis. I know it's expensive and to figure out how translate efficiently and optimally from sketchup to artlantis is a little confusing at first, but once figured out there's nothing as optimal. I say optimal because I'm weighing the time taken to render v/s the rendering quality. If you're looking for quick workable renders that is. There are some great hidden gems in artlantis which are not apparent at first.
If you're looking for truly photorealistic renders, then probably something else like mantelray etc...
Hello. I think since this thread was started, a lot has changed. Indigo is now a paid-for renderer, albeit an exceptional one. But there are also many others - for example, Piranesi is now available for Sketchup and the relative newcomer Shaderlight gives awesome photo-realistic results, but at about 20% of the cost of everything else. For ease of use and an established reputation, V-Ray for Sketchup is one of the best.
I'd definately recommend vray, it gives good resolution renders (I got 30MP on my netbook), but it allows you to make something emmisive, refractive, or reflective, with full conrols over that (volume fog, volume blur, colours, textures...)