I have 5 boxed and raided since Burning Crusade era and you are always a detriment to the raid until you master all of the fights. On some fights, you will always be a detriment due to the fight requiring fine movement or fine manipulation of buffs or debufs.

As some have already stated, Tier 11 had some great fights in them that were more difficult to multibox than previous tiers.

Once the novelty of it wore off I enjoyed multiboxing in raids much less as it is much more intense than solo playing, and can be very stressful. Mistakes you make will be compounded, and the actions of others will have to take in account your presence in the raid.

Now I don't box in raids if I can avoid it. Boxing in BGs, Heroics and Arena's disappoints fewer other people.