This item is bugged pretty badly, especially for paladins (or it got a huge buff I just hadn't heard about). Holy Shock, Holy Light, and Flash of Light all give double procs of the trinket effect as seen in this screenshot:
http://img341.imageshack.us/my.php?image=violetvt2.jpg Other paladin spells like consecration, cleanse, divine shield, etc all only give a single stack of the buff. I imagine it has something to do with the Illumination talent counting as 2 spells; I'll check if I respec to a build without it sometime. Spells such as Stoneform, Find Minerals, Sense Undead, Divine Favor, Divine Illumination all also proc the trinket, but only give a single stack as seen here:
http://img96.imageshack.us/my.php?image=violet2rr0.jpg Non-combat pets and mounts do not proc the effect (spamming a non-combat pet to get 20 stacks instantly would've been awesome). The maximum attainable stack is 20, as is listed in the spell data at
http://thottbot.com/?sp=35095 . I also verified this in game by landing 11 heals in the duration with double procs on a Scarab of the Infinite Cycle haste proc, and the final heal did not increase the stack above 20. In practice, I usually hit 18 stacks by the end of the effect, or 20 if I get a Scarab of the Infinite Cycle proc or throw in a Holy Shock or Hammer of Justice or Cleanse or something between Flash of Lights.
The mana regen gained by the trinket is actual mp/5. That is, it increases the amount of mana gained on each 2 second tick that you normally recieve and is displayed in your mana regen in the character sheet. It does NOT work like blessing of wisdom. The mana regen buff fades the second the duration expires. I ran through one of my combat logs to see how much mana this gives over a long period of time, and my results were as follows:
04:57:15.671 You gain Enlightenment.
04:57:17.281 You gain Enlightenment.
04:57:17.671 You gain Enlightenment (3).
04:57:18.187 You gain Enlightenment (4).
04:57:20.500 You gain Enlightenment (6).
04:57:20.890 You gain Spell Haste. <--- Scarab of the Infinite Cycle proc
04:57:22.125 You gain Enlightenment (8).
04:57:23.734 You gain Enlightenment (10).
04:57:25.421 You gain Enlightenment (12).
04:57:26.609 Spell Haste fades from you.
04:57:27.031 You gain Enlightenment (14).
04:57:29.046 You gain Enlightenment (16).
04:57:31.031 You gain Enlightenment (18).
04:57:32.953 You gain Enlightenment (20).
04:57:35.671 Enlightenment fades from you.
04:57:35.687 Enlightenment fades from you.
We'll assume that mana ticked up when the trinket was clicked, and each 2 seconds thereafter, so :15.671, :17.671, :19.671, etc. The stack quantities at each of these times were 3, 4, 6, 8, 12, 14, 16, 18, 20, 20. That means the mana gained from the proc would be ( 14 mana / 5 sec ) * 2 seconds * ( 3 + 4 + 6 + 8 + 12 + 14 + 16 + 18 + 20 + 20 ) = 677.6 mana. Cooldown is 2 minutes so that's 677.6 / 120 seconds * 5 seconds = 28.233 mp/5 assuming you used the trinket exactly every 2 minutes and got a haste proc every time. It ends up about 24 mp/5 without the scarab proc and an 18 stack.
I was only able to get up to a 12 stack (like others earlier in the thread) spamming Sense Undead or Find Minerals. Results for that were as follows:
06:36:07.328 You gain Enlightenment.
06:36:07.906 You gain Enlightenment.
06:36:09.437 You gain Enlightenment (2).
06:36:11.531 You gain Enlightenment (3).
06:36:12.625 You gain Enlightenment (4).
06:36:14.656 You gain Enlightenment (5).
06:36:15.968 You gain Enlightenment (6).
06:36:17.578 You gain Enlightenment (7).
06:36:19.625 You gain Enlightenment (8).
06:36:21.593 You gain Enlightenment (9).
06:36:23.015 You gain Enlightenment (10).
06:36:24.406 You gain Enlightenment (11).
06:36:26.218 You gain Enlightenment (12).
06:36:27.203 Enlightenment fades from you.
06:36:27.203 Enlightenment fades from you.
Using the same assumptions as above, my stacks at each tick were 1, 2, 4, 5, 6, 7, 8, 10, 11, 12. So 14 / 5 * 2 * ( 1 + 2 + 4 + 5 + 6 + 7 + 8 + 10 + 11 + 12 ) = 369.6 mana. 369.6 / 120 * 5 = 15.4 mp/5, again assuming you use it exactly every 2 minutes and reach the maximum stack value.
By comparison, Warmth of Forgiveness, an ilvl 85 epic from naxxramas is 10 mp/5 + 500 mana per 3 minutes on a clicky Use: effect. Thats 23.88 mp/5 for an ilvl 85 level 60 trinket. Pendant of the Violet Eye is a level 70 ilvl 100 epic that gives 28.233 mp/5 while bugged, used perfectly with a chance haste proc (more like 20 mp/5 realistically due to hitting 18 stacks max without a proc and it being pretty much impossible to use it EXACTLY every 2 minutes unlike Warmth of Forgiveness), and is harder to use optimally. I don't think I even need to bring up Mindtap Talisman for comparison. It's absolute garbage currently for a non-paladin at 15.4 mp/5 when used perfectly, assuming this is a paladin-only bug.
Pendant of the Violet Eye should have the double stack bug fixed, make find minerals etc not proc it, and be buffed like crazy. It should either be around 30 mp/5 from each stack instead of 14 mp/5 or it should remain 14 mp/5 and have a passive ~12-15 mp/5 or an equivalent amount of spelldmg/healing/int/stam/whatever in addition to the Use: effect. Or make the stack remain active after the buff fades long enough for it to actually be useful. On a side note, with the bugged double procs, it's pretty awesome with a Scarab of the Infinite Cycle proc plus heroism from the shaman I group with normally.