Arkel XM series - Very nice. No problems at all. Tough. Secure on and offroad. Easy on and off of the rack. No clearance issues for me. Can open them from the top or the side. Not waterproof, but are resistant. Combined with rain covers, my gear has stayed dry.

Jandd Mountain series - More difficult to put on the bike (requires getting your hand between the wheel and rack to cinch them tight. Hard to get tight enough to secure them for offroad/fire roads. No problems on pavement though. Extending tops can be convenient for when you're temporarily carrying extra stuff. Rears hold a lot of stuff! fronts are tight. Only accessible through the top. No problems with rain when used with covers. Covers don't fit as well as Arkels since they're sized to accommodate the extendability of the panniers.