http://www.youtube.com/watch?v=c7wTaW46gzA ERPT vs PT (standard Path tracing)
ERPT on the left, standard path tracing (PT) on the right. Light is coming in from a narrow opening, a scenario in which PT has a hard time to find light paths and converge, because it randomly samples the environment. ERPT shares properties with MLT: once it finds an important light path, it will sample nearby paths via small mutations of the found light path, so convergence is much faster.
http://www.youtube.com/watch?v=70uNjjplYzA Kelemen style MLT on GPU
Kelemen-MLT after 10 seconds of rendering at 1280x720 on a single GTX 470. The beautiful caustics are possible due to bidirectional path tracing+MLT and are much more difficult to obtain with standard path tracing.
http://www.youtube.com/watch?v=d9X_PhFIL1o&feature=related ERPT vs PT on GPU