The foundation of Searchlight is a fairly simple concept: use Leopard's built-in technologies to navigate, preview and download remote files. This means that from your computer (or from your iPhone), you have access to anything you've allowed Spotlight to index, including emails, calendar events, images, documents, and so on.
Using Quick Look for previews means you can quickly get thumbnail views and previews of a broad range of filetypes. It even recognizes some Quick Look plugins, allowing me, for example, to search for and preview EPS files. I can also search for snippets in source code and see a code-highlighted version with the Quick Look preview or the entire source by choosing the Download option.
Searchlight is a search-only interface; browsing through your folders isn't an option. This could be a deciding factor based on the usage scenario. For me, there are two reasons a Spotlight search is ideal. First, I use a tagging system and a limited folder hierarchy, so even when I'm at my computer I find everything through Spotlight or Spotlight-based applications. Second, the primary reason I access my remote machine and its large external drives is to search for either images or code snippets. My image collection is a vast array of fairly meaningless filenames, and if I'm searching for code I rarely know which file contains the snippet I want ... a Spotlight-powered keyword or content search is my best hope.
Searchlight deals very well with almost every type of file, including emails, events, images and all manner of documents. Searchlight offers good poster frame previews of movies, but one failure of every application I've tried has been movie downloads. Even movies which are already iPhone-compatible fail to play on the iPhone. Searchlight's authors tell me they're looking into this and plan to have a solution soon.
Searchlight's interface is one of its strongest features. Rather than describe it, I've put together a gallery which you'll find at the end of the post.
You can try out a demo version of Searchlight for free. If you're digging it, TUAW has 10 Searchlight licenses to give away to randomly-selected haiku entries. Feeling poetic? Just drop some 3-line (5 syllables, 7 syllables, 5 syllables) verse about remote file access (or a creative abstraction thereof) in the comments for a chance at a free license for the $99 application.
There are quite a few contenders in this arena, so watch for a TUAW Faceoff soon. We'll take a look at Searchlight, iGet, FarFinder, Back to my Mac, and ShareTool, as well as some homebrew options. Stay tuned!