Warning, the next 10s of lines may include maths algebraic in nature. If such mathematics make you uncomfortable, confused, or sick to the stomach, scroll down to the section marked "TL;DR". You have been warned.

Let's talk maths!

Multipliers:
SR = 1.3
Mangle (Shred) = 1.3
RaT (Shred) = 1.2
Crit = 2.0
Predatory Instincts: 1.1
Crit meta = 1.03
Savage Fury (Mangle) = 1.2

Shred:
(2.25 * x + 743) * 1.3 * 1.3 * 1.2 =
4.563 * x + 1507

Shred Crit:
(2.25 * x + 743) * 1.3 * 1.3 * 1.2 * 2.0 * 1.1 * 1.03 =
10.34 * x + 3414.86

With 50% crit, we can work out Shred's average damage per cast:
((4.563 * x + 1507) + (10.34 * x + 3414.86)) / 2 =
(14.903 * x + 4921.86) / 2 =
7.4515 * x + 2460.93

and now we can work out the average damage per energy:
(7.4515 * x + 2460.93) / 42 =
0.1774 * x + 58.5936

Mangle:
(2.0 * x + 634) * 1.3 * 1.2 =
3.12 * x + 989.04

Mangle Crit:
(2.0 * x + 634) * 1.3 * 1.2 * 2 * 1.1 * 1.03 =
7.06992 * x + 2241.16464

average the damage:
((3.12 * x + 989.04) + (7.06992 * x + 2241.16464)) / 2 =
(10.18992 * x + 3230.20464) / 2 =
5.09496 * x + 1615.10232

average damage per energy:
(5.09496 * x + 1615.10232) / 34 =
0.14985 * x + 47.50301

Shred vs. Mangle (DPE):
Shred: 0.1774 * x + 58.5936
Mangle: 0.14985 * x + 47.50301

TL;DR:
Okay, the point of all this maths above this point was to point out the VS. section just above. I think that it's pretty clear why Shred is a leaps-and-bounds better attack than Mangle, as both curves are linear and have no positive intercepts (that is, Shred will do more damage than Mangle for ANY value of X). This brings me nicely to the main topic here: the main problem with Mangle.

Mangle is a move which is easier to use in a PvP setting than Shred as it requires less set up to do it's maximum damage (you just use Mangle... as opposed to Shred which requires a bleed and to be behind your opponent to do its max damage). That being said, there is some penalty for using Mangle exclusively, in that you do less damage necessarily. However, the devs have made note on more than one occasion that they do not like how bursty Shred is in PvP AND they don't like the positional requirements (they think that that should be some PvE requirement on boss fights for maximum damage). However, I am still looking at the damage per energy and seeing Mangle come out below Shred, and I can still reasonably effectively use Shred in PvP and excel.

The problem I have is against teams who have a truly deep understanding of Feral mechanics and know how to exploit them. For instance, we faced an AMAZING pally+rogue team this season (ended up at ~2300 because they couldn't faceroll the better pally+dk teams). Each of them knew exactly what to do when I focused them: the pally would HoF the target and they would strafe to avoid me getting "behind" them. This works extremely well and completely neuters my damage output. However, I am not spec'd for Mangle-spamming, and if I were, I wouldn't put out enough damage to pressure most teams most of the time...

THIS is "The Mangle Problem": it doesn't do enough damage to spec strictly for it, but it does let you do damage against the teams who can take Shred off the table.

Bornakk said (in a post started by Deep that dealt with this topic head-on) that they would likely implement something to make Mangle more appealing in PvP, while still doing less damage than Shred, but not so little damage that you would never spec into it. Incidentally, this is the same thread that Bornakk also semi-confirmed my suggestion for Glyph of Primal Gore, which would lower the coefficient of Savage Roar to 15-20%, but allow Rake to critically tick as well as Rip, saying that "Something similar to this was discussed recently, but I wouldn't expect it to happen in 3.1" (source). Also noted concerning Mangle, "One example would be to make Rend and Tear include mangle in its damage bonuses" (source).

WELL NOW, if RaT includes Mangle in its damage bonuses, then we might be on to something. Time for more maths, if you don't care, just scroll to the next bit of paragraphs.

Proposed Mangle:
(2.0 * x + 634) * 1.3 * 1.2 * 1.2 =
3.744 * x + 1186.848

Proposed Mangle Crit:
(2.0 * x + 634) * 1.3 * 1.2 * 1.2 * 2 * 1.1 * 1.03 =
8.483904 * x + 2689.397568

Average damage:
((3.744 * x + 1186.848) + (8.483904 * x + 2689.397568)) / 2 =
(12.227904 * x + 3876.245568) / 2 =
6.113952 * x + 1938.122784

Average Damage per Energy:
(6.113952 * x + 1938.122784) / 34 =
0.179822 * x + 57.0036

Shred ADPE vs. Proposed Mangle ADPE:
Shred: 0.1774 * x + 58.5936
Mangle: 0.179822 * x + 57.0036

OOOOOOOO... now we're on to something. With the proposed change to Mangle, we get a few things:
1) The average damage for Shred is better than Mangle in almost all cases of X.
2) The highest burst damage for Mangle is still 20-30% less than the highest burst damage for Shred.
3) The DPE for Mangle is better than Shred at 50% crit.

That last bit might be a problem... something tells me that Blizz doesn't want ferals using Mangle exclusively in PvE, but only PvP. That is no problem, they could pretty reliably change the coefficient for Mangle on RaT from 20% to 15% (or whatever makes the maths work out properly), the point is still the same: you could do SOMETHING to mangle's damage and it would be a better PvP situation for ferals.

My only problem with it would be coming up with a spec. My current spec has everything I need for damage, survivability, and utility (in terms of shapeshifting), but if Mangle becomes the PvP attack du jour, then I am at a loss for what to drop. I suppose it would have to be some from Protector of the Pack, with this spec. On the other hand... they could simply make a glyph that improves Mangle's damage and unlink Rend and Tear from Primal Gore... that would make this spec the uber spec... and that would be VERY nice for PvP.

I'm hoping for something in that regard, but until then I'll keep using Shred. Deep's trying out a Mangle-spam spec, it seems, so I'm curious to hear his thoughts on the matter. Season 6 starts Tuesday, so I'm getting excited again.