Most of the time they won't give you any benefit, but where the terrain suits it can be quite useful.I haven't even seen a flank march, let alone a double. Every time I'm tempted I look at the odds and the urge goes away. Are they common in 15mm?
At Usk Simon and I flanked marched a couple of weak BGs to end up behind a fortification defended by poor MF.
We took that BG down and also his baggage which helped us on the way to a big victory.
(we did manage to make our arrival roll first attempt)
If you take only mounted BGs led by an FC you have a good chance of getting on the table early.