"Riggy Assist" preamp from JuicedLink gives your camera clean sound, while syncing the sound directly to your shots. It's best used for ENG, documentary, or indie film shooting, when speed in post is a priori-tah.
The sound is clean and quiet, and slightly better than using a dual-system recorder like the Zoom H4N. Using a preamp like this is definitely better than using your camera's amp.
Riggy Assist (RA333) to my Canon 5DMKIII, and turned the camera's audio gain all the way down to one click above "off." I cranked up the gain on the Riggy Assist, and then used the camera's meters to set the proper levels. JuicedLink has several models to choose from, and their flagship model, the RA333, ($469) has 3 XLR inputs (e.g. 2 lavs and a shotgun mic). It also includes a headphone jack for monitoring your audio, and visual meters. Depending on your camera's AGC situation (5DMKII, 60D), the Riggy Assist can disable it with a switch option. You can also output in stereo, or mix all three inputs down to a mono output.
< The underside of the Riggy Assist gives you many options (or switches) for minimizing the drain to the one 9v battery that powers the unit. However, the lack of an external power jack is a bummer. The RA333 also includes an audio output option that will record one track at regular levels, and the 2nd track at -16db to prevent any blowouts to the main channel. This is a tried and true practice of professional audio mixers on film sets that gives them a safety track in post. It's a great feature to have in this small preamp box, and will save your ass in post, especially when your actor/subject suddenly decides to scream their dialogue. You can also easily ride the levels, as needed.
OVERALL: I highly recommend the Riggy Assist if you're looking for clean sound (i.e. signal to noise ratio), and the ability to automatically sync your sound to your camera dailies. There's no need for a slate clap, or 3rd party syncing software like PluralEyes, in order to sync up the sound to picture, which makes the whole post process that much easier and faster. I've been using other preamps in the field, like the Beachtek, but can't wait to use the Riggy Assist on future productions. It's perfect for a one-man band scenario. I'll update this post if any problems arise, but the Riggy Assist preamp by JuicedLink is a solid solution for clean sound to your DSLR. JB's score: A-