I ran a 5 hunters group to level 70 and swapped in a druid for healing between 70 and 80.

I found pet control to be rather easy as I used identical pets. It really felt like controlling only one pet (with 4 times shorter cooldowns). I could run instances with just 5 hunters pre-70. The druid made it possible to run Wrath instances, but I could not do any heroics. The pets would just not tank well enough.

Bottom line : Multi-boxed hunters are very fun, but I would bring a healer along for more versatility.