I play mostly everything from clean jazz and all blues styles, through to heavy rock.
It gives more depth and a much faster response than my other cab, which has two Celestion G12T-75s in it. Never liked those speakers, didn't flatter either the amp or my guitars, and had a horrible farty sound around the upper midrange.
The blackbirds in this cab seem to flatter everything. Totally different beast, and I'm a fussy git when I've got the chance to be! My guitar plays like silk now too, no more seeming to resist the notes I want to play, coz the speakers gel with the player.
There's bound to be people who are not so keen on them, it's personal taste. They're right for me though.
I dunno what Engl use and how the wattage and ohms are in your cab, you may need to think about the changes you'll make if you replace them one by one. Others on here or the shop themselves will know more about speaker matching in cabs.
The cab itself seems to be good, yes, it's fairly compact but projects well and there's a special panel at the back you can remove if you want to port it and spread the sound more. Haven't had it long so I can't say how tough it is for the road yet etc.