Got a few test games in with Beastmasters at the 2500 point level. I played a IG meltavet/chimera spam/ vendetta spam list and a 2500 vulkan/lysander heavy termi list.
The first game was annihilation - pitched battle and we let IG go first. 9 chimera's and 7 vendetta's. The wound allocation on the beast squads makes the beastmasters extremely resilient. He could not make me take a leadership test. Dice were a bit above average for my opponent while mine where average. Having a Haemonculus with each beastmaster squad giving FNP made all the difference. (typically a Chimera would fire its multi-laser and HB hitting with half of them and wounding with 2 or 3. allocate S6 to the clawfiend so it can get a FNP and the other S6 to kymera for the 4++ hvy bolter shots go on Razorwings for a FNP and 5 wounds before the model gets removed.) Vendetta hits go on kymera for the 4++
By turn 2 (the Haemi's left their pain tokens and joined Wrack squads) my beast masters were in my opponents lines stunning and immobilizing 3 chimera's my lances/blasters (13lances and 12blasters) had completely failed except to stun the Pyscher battle squad's chimera (meaning if they got out they would be charged by beastmasters or wracks.)
Essentially he destroyed my first beastmasters after they caused havoc in his lines and the second unit of beastmasters came in along with wracks and finished the job, all supported by ravagers and trueborn blaster boats. Oh and my wyches were able to multi-charge two squadrons of vendetta's and slowly kill them off with haywire grenades. My dark light weapons then finally kicked in giving my a massacre.
Tactical Analysis - Target prioritization is a bitch when faced with a combined arms DE army that has beastmasters. The beastmasters can absorb a ridiculous amount of fire-power and play Thunderwolf style wound allocation games. If they are not dealt with they are in your lines on turn 2 or 3 at the latest. I see folks talking about using them out of WWP, however I think that the beasts better serve the army on the board absorbing fire and woe to the player who ignores them.
Disclaimer - So far we have not tested this against more than one vindicator in a list so the beasts may not perform as well against lots of blast weapons. However both of the direct hits that the Vindicator laid down did insignificant damage (3 models total)