I paid $3.99 for one (the Burger King I visited wasn't participating in the two for $5 deal).
Unsurprisingly, it tasted much like a Rodeo Cheeseburger but slightly meatier due to the beef patties being slightly wider than the long sesame bun.
There's a lot of barbecue sauce going on but I liked the balanced blend of spicy, sweet, smoky, and tangy (they use Sweet Baby Ray's from what I can tell) and there's enough beef and onion rings to carry through.
The patties had a slight char to them and were decently moist for a major fast food chain.
The cheese was relatively harmless and nicely melted. I'm not a fan of cheese and barbecue but it works out okay.
The onion rings were relatively crispy, easy to bite through (BK's rings are actually formed onion rings rather than whole onion slices), and add a complementary sweet flavor.
Overall, I enjoyed the Extra Long BBQ Cheeseburger about as much as I enjoy the Rodeo Cheeseburger, which is to say it's quite good. The few ingredients work very well together with very little unnecessary bits (even the cheese isn't bad here). With the Rodeo Burger currently available for only a buck though, it's not a good value unless you grab two (at most locations).
Calories - 590 (from Fat - 250)
Fat - 28g (Saturated Fat - 11g)
Sodium - 1080mg
Carbs - 62g (Sugar - 14g)
Protein - 22g