Many tile sources carry variations, but some look more natural than others. After a bit of local scouting, I found that The Tile Shop's Coteau Rock tile most closely mimics the natural variations in size and texture found in clustered groups of rocks occurring in nature. Unlike the prefab river rock tiles of ten years ago, the more common trend today is to lay mesh sheets of un-grouted river rocks in the same manner that you would other mosaic tiles, and then grout after adhering. Most rock tile require the same standard grout as other types of tile, but it's worth asking your manufacturer for advice since, according to a representative at The Tile Shop, some companies advise adding silica sand to the grout.
If you are considering the look, be sure to research the types of rocks or pebbles that you are using for your specific purpose. Some pebbles that are suitable for light-use areas like back-splashes will not hold up in high traffic applications like entranceway floors. For instance, Home Depot's Merola Tile River Stone is specifically intended for light traffic areas, and they caution consumers not to use in areas with "direct access from outside, no scratching dirt can be present."
Now, do any of you river rock tile layers want to add your two cents?