Bidirectional A* (A-star) Search

I'm implementing a bidirectional A* search (bidirectional as in the search is performed from both the origin and destination simultaneously, and when these two searches meet, I'll have my shortest path - at least with a bit of extra logic thrown in).

Does anyone have any experience with taking a unidirectional A* and bidirectionalising(!) it - what sort of performance gain can I expect? I'd reckoned on it more-or-less halving the search time, at a minimum - but may I see bigger gains that this? I'm using the algorithm to determine shortest routes on a road network - if that's in any way relevant (I've read about MS's "Reach" algorithm, but want to take baby-steps towards this rather than jumping straight in).