That said, let's check their stats:
WS 4, BS 2, S 3, T 4(5), W 2, I 2, A 2, Ld 7, Sv 4+
Mob Rule seems a worthless add-on, since you're limited to no more than 5 models per squad.
Furious Charge and Waaagh! are nice to have, though standard for the army.
Deffcoptas are considered Jetbikes. Hopping over terrain is sweet.
Also, they benefit from both the Scout and Hit and Run rules. Everything here is pointing towards a highly maneuverable unit. Let's check out their equipment:
Twin-Linked Big Shoota. This is the standard equipment. It sounds nice, but Warbikers have a similar gun (although not TL) and are about 10 points cheaper per model.
Twin-Linked Rokkit Launcha is a 10pt upgrade per model. Despite how expensive this makes the models, I like the idea of highly mobile TL Rokkit Launchas.
Kustom Mega Blasta is a 5 pt upgrade. This is basically the same as the above Rokkit Launcha, just not Twin-linked ... which now means less guns hitting their targets.
Bigbomm is a 15pt add-on. It looks like a huge waste of time and points, as you have to fly over where the template is placed, it will scatter, and it's ony S4, AP5
Buzzsaw is a 25pt add-on. This is essentially a Power Klaw. Something to keep in mind here, is that you're paying the same 25 pts that a Nob pays for this upgrade. A Deffcopta's Strength is 3 (making this an S6 Power Klaw) vs a Nob's Strength of 4 (an S8 Power Klaw). Same price for a worse version. Save the 25 for a Nob, they can do it better.
OK, so we've covered all of their options. Lets look at how they would be used.
With the standard gear, they seem like overpriced Warbikers. More maneuverable, yes, but also limited to very small mobs of up to 5. The mob limit of 5 makes them highly vulnerable to any kind of assault, regardless of Hit and Run. The part that interests me is the TL Rokkit Launchas. That upgrade makes them 45 pts/model, though. 5 'coptas would be a 225 pt mob. While the idea of 5 TL Rokkit Launchas hitting side/rear armour ... I'm not sure I'm convinced that they're worth the points. Compare them to Warbuggies, though (another Fast Attack option); A Warbuggy with a TL Rokkit Launcha is only 35 pts/model. That's 10 points cheaper.
Here's a differing opinion on the buzzsaw/rokkit take. His answer to the cost? Take sacrificial squads of 1-2.
"Buzzsaws are crappy if you're using them as anti-infantry weapon. The real point of a buzzsaw is to be S7 versus a vehicles rear armor during assault. With few exceptions (land raiders, monoliths), that gives you a 50% chance to get a pen with each hit. And if you get first turn, that means you'll probably get 1 or 2 pens per deffkopta on any vehicle you can reach.
I run units of 1 or 2. They are sacrificial units. If I go first, they scout-turboboost, then go for the side/rear armor on a transport or gun platform, and aim to assault that vehicle. If I go second, they outflank and do the same. Even if they don't actually kill anything, the opponent has to stop and deal with them, giving the rest of your orks time to get to optimal charge range.
My most recent game had a great example of this. At 2000 points I had 3 units of 2, all with rokkits and buzzsaws, up against a Blood Angels army with 2 Baals, a Whirlwind, and a couple of Rhinos. I got to go first, and on that first turn, between the 6 koptas, they killed a Rhino and stunned or shook every other vehicle and assaulted a the only tac marine combat squad with a long range anti-tank weapon. By turn 3 the Whirlwind was dead and neither Predator ever got to shoot before the battle wagons with deffrollas arrived to finish them off. One Kopta even survived long enough to help kill off most of the combat squad of Tac marines.
All 6 deffkoptas died, but in doing so they kept everything that could have targeted my Trukks and Battlewagons early game until those my troops were delivered into combat. My Nobz, Mega Nobz, and Shoota boyz all got do exactly what they do best and as a result I tabled my opponent."
Most of this is speculation over what I see in the books. To be honest, I haven't actually had the opportunity to field them much (just finished painting them). Have any of you guys used Deffcoptas? What have your experiences been with this unit? Leave a comment, I might just add your input to the post.