A rather simple guide or rather sort of analysis, maybe some people will find this helpful. Taken from: http://devizzlive.blogspot.com
Some people asked me, when does crit damage become more valuable than crit chance and vice versa. Well, both stats are affected by each other and their actual value might differ depending on your personal stats.
By default, every class has base critical hit chance of 5% and critical hit damage of 50%.
Now, assuming that you increase them both proportionally, their value will remain the same. So, to make them most effective, you stats should progress in the following way:
10% Chc and 100% Chd
20% Chc and 200% Chd
30% Chc and 300% Chd
40% Chc and 400% Chd
50% Chc and 500% Chd
and so on...
Now lets take a look at the following example:
Lets assume that your current crit chance is at 10% and Chd at 100%.
Increasing your Chd by 200% will give you the same net gain in DPS as increasing your Chc by 20%
On the other hand, if you will increase your Chd by 100% and your Chc by 10%, you DPS will increase by TWICE as much.
And thus we are arrive at a simple conclusion: By balancing your crit damage and crit chance, you will gain more DPS than by simply stacking one of the two stats.
The above conclusion might of course vary from class to class, depending on what buffs your class has. Remember to take them into account when deciding whether to go for Chd or Chc.
Also, when you are deciding whether to roll Chd or Chc on an item, be sure to know what max rolls you can get on the said item. For this purpose, I advise you to make yourself familiar with the following website: http://www.d3maxstats.com/ (it does the job quite nicely and is relatively easy to use, with clean and simple user interface).
For example, you might want to increase your EHP by getting more vitality. If you have 50% Chd or 6% Chc on your ring, you should probably not replace any of these stats with vitality. Instead, you're better of getting rid of your weapon with 30-35% Chd. If you are using a weapon with the following stats: a socket, 650-750 primary stat and 30-35% Chd, you probably have no vitality on it. If your other weapon with potential vitality has same weapon DMG, you will gain more DPS by equipping that weapon and replacing your 30-35% ChD with 650-750 vitality. This is just an example to show you how important it is to know highest possible rolls in Diablo on all types of items.