I have a set of Enves. I also have a set of reynolds AM carbon rims, and a set of Crossmax ST's, all in 29 inch.
Are the Enves any better than the Reynolds? Yes - much. stiffer, lighter, stronger. Are they better than the Mavic's? Marginally, they're wider so give the tyre a better profile and are a bit lighter.
For me, having a decent set of stiff wheels on a 29er makes the bike. I've ridden one with standard OEM rubbish and they were just that - rubbish. Made the handling vague, felt sluggish and nasty to ride. Do you need to spend Enve money to stop this happening? No, of course not. A set of Flows would probably do almost as good a job.
I got the Enve's because they look cool and at the time there wasn't really anything that compared. I can't comment on the LB rims as I've not ridden a set but I'm sure they're very good. I think hating a rim because its expensive is a bit of a joke - you don't have to buy them, there are alternatives.
You spend your money, you takes your choice. For the record I don't think they are perfect but I've found them to be very good. The reynolds on the other hand were similar money and are rubbish. And this is my second set (I've had them in 26inch too) and neither were great. Bought as complete wheelsets, these have the i9 hubs which I've also found to be bobbins - bearing life is laughable.
Cost aside, a set of Enve's on Kings is in (my opinion) the optimum wheelset. Look good, light enough, strong and stiff.