I've seen it. Barkskin is strange in that its bonus scales by level, and it can be used on ally. Both of these are important factors, because it typically won't be good for the Druid to use it on himself, and since the power ends up more than twice as effective by epic levels (figure Con 16-18 at level 2 and Con 24-28 by level 30), it is likely to be somewhat underpowered at the level when you get it to avoid scaling issues.
That said, let's go with a +4 Con bonus at level 2, and assume our metric is average attacks blocked per encounter. Then Barkskin stops 0.2*the number of attacks against AC for the round. If you can get 3 attacks (vs. AC) against the protected character on average, that's comparable to Shield. If you can only get 1.5 attacks vs. AC, not so much.
Having a fighter with Come and Get It (level 7 encounter), for example, makes it much more likely you'll get good use out of the power. In general, I think Barkskin is going to be a little weak at level 2, but it will scale nicely.
