I freely admit: I love Rawr. Absolutely love it.
If you've not played with Rawr and are a feral druid, moonkin, warlock, ret paladin or mage, download it now. It's the best optimizing tool I've yet to find. It is not the best optimizer out there; there are better spreadsheets that do a better job of modeling DPS output than Rawr does, especially on some of the newer models. But what it does, it is stellar at.
For starters, everything is usable with easy interaction and clicking. No web access is needed and it performs faster given that. It also does real-time optimization and evaluation, so that as you change gear you can see immediately what effects that gear has on your items. It incorporates effective health, it incorporates raid buffs that are permanent and temporary. And the optimizer that will find the best gear for you based on what is available to you is stellar.
For me, this was very useful because it showed me I could have more health and avoidance by going for the vindicator bracers instead of the vindicator leather boots, which was the original plan. My original plan was to get the badge bracers, the pvp boots, and gem/enchant for stamina. It turns out that it's better to get the badge boots and the pvp bracers and gem for defense; between the extra stam and armor from the boots and some of the socket bonuses, enchanting for def works out best. Not significantly - we're only talking maybe 300 health and 200 armor - but every bit counts, and it does mean I can actually think about getting the Treads of the Den Mother without destroying my kit.
It's a good thing I did this math early as well, as finally my staff dropped. It has much more stam and no defense, which meant juggling around a lot of things to get enough def/resilience to get uncrittable. Thanks to Rawr, it wasn't hard to figure out which I should do. While the itemization of a feral druid is...well, frustrating, this makes it a lot easier.
What's also interesting about Rawr is doing evaluations both unbuffed and with raid buffs. When you take into account raid buffing, agility becomes much better as your stamina goes further and further up. Shifting Nightseyes become almost as valuable as solid stars. When you start chain-chugging ironshield pots, the value of a lot of equipment changes drastically as well.
What Rawr points out quite quickly is how good the set bonus for Tier 4 is. The 2-piece isn't anything Rawr cares about for tanks (at least not yet), but the 4-piece bonus is a huge value by itself, one that quickly overwhelms almost any other bonus. That makes it a bit tougher to get rid of it, even though it becomes a lot more important to get rid of those pieces as upgrades become avaialable; you simply need the threat output.
That's the only real drawback at this point; there is no 'maximize for threat' option. Or even a consideration of how valuable threat is. I realize that this is totally subjective; how can you evaluate the quality of expertise relative to dodge? Okay, bad example. But it would be good to be able to evaluate how much threat one would do in a certain buildout and juggle as you go. Right now the brooch of deftness is one of the worst trinkets for tanking as judged by Rawr but this is clearly wrong; the hit/expertise helps threat immensely as well as avoiding more parry gibs. The lesson is simply that Rawr is a great tool but you must be aware of the limitations it has.