Using flash nearly everyday now, it's important I can rely on a successful fire rate. This meant throwing away the old eBay Cactus V2 triggers and moving to something more reliable. First choice may have been the pocketwizards, but now owning 2 speedlites a set would of cost nearly £700! The ST-E2 is cheaper and works on an infra-red beam (and therefore line of sight), but from my trials i'm yet to find any problems with range or triggering abilities for my current needs.
The ST-E2 acts as an AF-Assist beam which is great for shooting in lower light conditions and when I decide to upgrade to the Pocketwizards, the ST-E2 is still a handy addition. Attaching the ST-E2 ontop of the Pocketwizards will allow you to still use the 'ratio' function allowing you greater flexibility with your speedlites.