Basically your Action Point & Health ratio per turn remains the same when you pick Glass Cannon.
If you normally die in 4 hits with 10 AP that's 40 AP, compared to 2 hits with 20 AP each turn. The trick is of course surviving at least 1 round;)
Dispite no actual net gain in AP, the main benefits comes from initiation (if you can take out someone right away it matters, but then they can do the same probably) or perhaps more important: buffing.
Glass cannon allows you to buff more in 1 turn, thus better making use of the remaining "buffed" action points.
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The downside... well, 1 less talent point for starters!
Also, for Hard mode I believe they use a mechanic where the -25% health is added additively for a total of -75%. Which leaves you with only 25 HP for every 100 HP as opposed to 37.5 HP. An odd choice in game mechanics that actually makes you lose 33% more HP than it should IMO.
When you think about it in reverse, you only gain 25% HP for every X buff/attribute point whereas someone without Glass Cannon gets 75%. That's a triple increase while you expect only a double increase like the talent advertises..
This only applies to Hard mode though.
Bottom line is, if you think you can survive at least 1 round with your glass cannon and make proper use of the improved initiation or buffing then it can be worth it.
