Fleet D'D has an excellent blend of defense, science and firepower. It also has versatility. The only thing it really lacks is turn rate, but liberal use of battlecloak mitigates that problem.
Agreed here. The D'D gets a bad rap because the free version is a reskin of the T4 Galaxy and comes right after the T3 Mogai because Cryptic was too lazy to give the Romulans a full range of warbirds. But it's hard to beat something that has access to LCDR powers in all three disciplines.
D'deridex is a very nice, all-purpose ship that can take a beating while doing a little bit of everything all at the same time.
Haakona is a little quirky, but it's a very flexible ship - you could run it as an 'escort' with a personal heal-boat, a control science ship with an attack pet, or just an eng-heavy tank if you wanted to.
for PvE, I can say from experience that the fleet D'Deridex can be set up as one of the best beam boat tanks in the game, with the right equipment. If you haven't picked up the valdore for the console, it's worth it.
When I say tank, I mean a blend of damage output and threat to get aggro from multiple baddies simultaneously and the ability to mitigate the incoming damage from those same multiple baddies at the same time. It can be done very well on the D'Deridex without needing an A2B setup. The allure of damage output from the scimitar is the only thing that got me out of the D'Deridex, which I run on the same toon which makes the scimitar and pseudo tank too, but I'm considering going back to her for kicks.
Anyway, I can't recommend buying anything new right now, until we know more about the new stuff that is coming real soon and how our current stuff will fair to it.