Re:
Same basis as a recent discussion about brand-new XT hubs..
The ISIS BB's have a much larger axle spline compared to Square Taper, and even larger than Octalink.
To fit the larger axle in a BB that's the same diameter, the bearings have to be smaller, so they are more at risk of getting damaged as they are smaller and weaker.
New Hollowtech and other external BB's get around this by using a massive spindle but sitting the bearings outboard of the frame BB shell. This is better for the bearings, but leaves them more exposed to the elements, particularly if you ride in muddy, gritty soil.
Personally I've moved away from ISIS and won't use it again after the experiences I've had. (Not all ISIS cranks fit all ISIS BB's properly, despite the 'IS' part of the name standing for International Standard). They need unusually high torque to tighten up the crank bolts to stop them coming loose (which in turn can damage the splines if they're not correctly aligned) and they have a habit of creaking like an old door hinge while powdering their bearings.
Advantage of ISIS is that they are about the lightest BB/crank weight you can get a lot of the time, and they are stiffer than many SqT setups (how much of this is simply age advancing materials I don't know), but I'd stick with Octalink or SqT for the majority of off-road bikes with a conventional BB shell.
