Depends against who your are using the transformation and how.
Pre-combat : don't even try because you can't stealth and you won't have the sustain to make a comeback.
Mid-combat : you can prebuff and use the invulnerability frame to your advantage to block a big hit like meteor or fear players in melee range to deal devastating damage while they are CCed. If they survive they end up in a situation where they are either forced to retreat or trying to sustain (which is almost impossible to do with the huge DPS we've got).
For the purpose of fighting multiple ennemies, just forget about it. You don't have enough sustain (even with claws of life) and will just consume way to much stamina to remain long enough in a fight. You might on the other hand kill a guy or two if you get lucky with the crits and hit ennemies with the fear from the transformation.
WWs aren't that bad realy, they do have some good hard CCs. But their lack of burst healing couple with their lack of soft CCs (like snares and roots) makes it very hard for them to stay in melee range with ennemies.
werewolf transformations is only useful in two situations. The best use being solo PvE delves, soloing public dungeons, and soloing quests which have 4 to 10 minute periods with constant fighting and no dialogue interruptions. Because of this a WW transformation choice is excellent for a second character playthrough because you know which quest chains and areas it is worthwhile to slot transformation.
The only other useful situation is 4-man dungeons when you have 2 damage dealers both with werewolf, or 3, and a templar tank. With full point investment and all morphed werewolf skills, including one pack leader... they can wreck non-veteran dungeons and several of the veteran dungeons.
I don't use it for pvp, because you get stunned going into the transformation, and stunned when it ends. Also... 90% of the time when I tried to transform in pvp, any damaging attack seems to knock me out of the transformation. And... I just waste 300 ultimate, and get no WW transformation. All I get for it is at best... stunned and no ultimate, and at worst dead.
Once you are in WW form, it's not bad. Good hard cc etc. Even that isn't that great though, when you're missing out on any class skills that could be damn useful for you at that point. If they made it a toggle, I'd probably use it. But having that *** run out on your in a fight is asking to be killed.
Fought a dueler who tried to WW on me. It was one of the shortest fights I had, and it wasn't me who was dead at the end.