I have the B2.1u.
I'd have to say that I like the expression pedal's features a lot.
You can assign the pedal to anything you want. Wet/dry mixes, effect rates, depth... And you can use the pedal to shut off the effect you assigned it to.
For instance, say, you're playing a patch with an Ampeg sound, and in the middle of the song you want some chorus. With the B2, you'll have to save a whole new patch next to your current one, but this time with the chorus added.
With the exp. pedal, instead of saving a whole new patch, you can assign chorus to it and use the pedal to turn it on. Full toe down postion activates a button for this function.
Pretty handy.
Plus the whammy function is cool with the pedal. Smooth octave up.
The exp is definitely a feature which adds a lot of flexibility and convenience in my opinion.