They are definitely worth it. They are lightweight, rock solid, and cheap compared to other quality BUIS. I also like being able to flip them up with the push of a button, as opposed to manually having to pull them up. Expect to paya around $80-100 for the pair. If you find them much cheaper than $70 for the pair brand new, then they are probably Magpul's PTS sights, which are not as high quality, made for airsoft guns, and as a result will not hold a zero for long. If you compare this price to the other high end brands (Troy, Midwest Industries, YHM, GG&G), then you will quickly realize that the MBUS are very cheap. Just a front or just a rear sight from these companies will cost you $100-120, and they work just as well as the MBUS.
Bottom line: As with all Magpul products, the MBUS are most definitely worth putting on your AR. You will not regret it.