Here is a simple guide of which berserk you'll want at certain PD skill levels.
The formula for attack rate is: base attack rate divided by attack speed boost. For normal heroes with 3/5 zerk we get 1000/1.2=833.33, not 800. Attack speed increase is not the equivalent of attack rate decrease.
Without PD any zerk below 4/5 is useless (on 1000 ms heroes). That means you need either 4/5 for 5/5 to see a benefit. Berserk at 4-5/5 will lower the attack rate from 1000 to 800 ms. This is why I have been so critical about zerk.
At 7/9 PD you'll want 3/5 zerk to see a benefit after the first proc. Your hero will go from 1000 ms to 600 ms just like that. It will take another 2 stacks to finally reach the 400 ms barrier. This isn't too bad for most game modes, but I hate that it relies on having 1 pd celebrate active at all times.
At 7/9 PD with 4/5 zerk you'll be at the 800 ms without a celebrate active. This is good. After celebrate you'll be at 600 ms just like 3/5, and again you'll have to get 2 more stacks before reaching 400 ms barrier.
This means you'll want 5/5 zerk on a hero for maximum benefit from a 7/9 PD. No pd celebrate = 800 ms. 1 celebrate = 600 ms. 2 celebrate's = 400 ms.
At 8/9 PD we get slightly different results. A 2/5 zerk *should* reach the 600 ms barrier after the first proc. That means it will jump from 1000 ms to 600 ms just like the first example at 7/9. Yet again it will take 2 more stacks before reaching the 400 ms barrier.
With 3/5 zerk and 8/9 PD you may get better results. The hero should jump from 1000 to 600 ms after one pd celebrate. The second celebrate should reduce the attack rate to 400 ms. This is not bad.
Since 3/5 reaches 400 ms after 2 stacks you'll want at least 4/5 to take advantage of the boost when celebrate is not active. Your hero will jump from 800 normally to 600 to 400 with each celebrate stack. At 8/9 PD you'll want at least 4/5 berserk.
Zerk 5/5 is no better than 4/5 at 8/9 PD.
With a maxed out PD you'll get better results with berserk. It only takes a 2/5 zerk to reach 600 ms and 400 ms with each celebrate stack. Your hero will still start out at 1000 ms and jump directly to 600 ms, but the benefit of 3-5/5 is non existent while celebrate is active.
At 9/9 PD you can benefit from 2/5 zerk, but I still advise having 4/5 or higher because you get the 800 ms speed while celebrate is not active.
A maxed PD is still useful for boss fights if you have no zerk because it is able to reach the 200 ms cap with just 4 stacks whereas 8/9 or lower will require 5 stacks, or a berserk attached to PD himself.
-Berserk is OK at 4/5 or higher with no PD, or a PD with 6/9 or lower. -7/9 PD requires 5/5 zerk for maximum utility. -8/9 PD requires 4/5 zerk for maximum utility. -9/9 PD requires 4/5 zerk for maximum utility; however, 9/9 PD without zerk will reach the 200 ms barrier after 4 stacks while 8/9 requires 5.
With this in mind, you'll likely want at least 4/5 zerk or you'll be better off re-rolling to a different talent. Remember, though, some lower level berserks may allow you to succeed in boss fights, but that's very situational.
-A ninja with ANY BERSERK LEVEL will receive no speed boost. There are only 2 situations where he will benefit from the berserk skill: 2x 7/9 pd stack with any level zerk will lower to 400 ms instead of 600 ms, and a 3x 9/9 PD stack with 3/5 or higher zerk will reach 200 ms instead of being stuck at 400. These situations are so particular that it makes berserk basically the WORST TALENT on ninja. -Heroes with slower attack rates, and heroes with attack rates on odd-beginning numbers will benefit more from lower level berserks than normal 1000 ms heroes. -Reaper can benefit from 4/5 zerk or better. For HBM purposes I'd suggest 4/5 berserk with a 9/9 pd because he will reach 600 ms normally and reach 400 ms attack rate after a single PD proc. That could be epic for tanking dragons/etc. At 8/9 PD you'll want 5/5 and no less to get the same results. Overall, I still prefer sprint, life drain, or revive. -9/9 pd needs either: a) a cupid (unknown skill level), b) berserk 2+ , or c) tanking the boss, to reach 200 Ms at 4 stacks. A 9/9 pd with none of the previous modifiers will be stuck at 3 stacks and will stay at 400 Ms. EDIT: turns out you need more than just berserk on PD to reach the speed cap. He will still require some external energy source.
The main reasons why the attack speed change affected us negatively:
-You need at least 4/5 zerk to see a benefit without PD stacks. -Heroes do not reach 900, 700, 500, or 300 ms intervals, which means you need certain combinations of zerk and PD to see true benefit. -Reaching attack speed of 400->300->200 needed for perm-stunning t3 boss became significantly more difficult.
Source: FC's when is berserk useful