I did a test. First, my character stats.
Level 42 Druid on Kargath server. 171 spirit.
I had a +1 spirit enchantment on my bracer, which would be the tool for my experiment to enchant it +3 spirit.
I would cast spells until my mana was close to 0, then when my mana started ticking (or regenning) I would note them and compare results.
+1 spirit enchantment test :
86, 138, 189, 241, 292, 343, 395, 446, 498, 549, 601
If you compare the amount of mana regenned between ticks, 5 times out of the 20 seconds (2 seconds is a tick) it only regenned 51 mana, the rest 52.
+3 spirit enchantment test :
10, 61, 113, 165, 217, 269, 321, 373, 424, 476, 528
With 2 extra spirit, my mana regenned between ticks twice of 51, the remainder of times 52.
From this, I gained an extra 3 mana in 20 seconds with 2 extra spirit, which is equal to 0.075 mana per second.
I also did a secondary test with the +3 spirit, and I only gained 51 mana once, instead of the last test of twice.
RESULTS : 1 spirit = 0.075-0.1 mana per second.
Of course, this is outside of battle, and without casting spells during the ticks. Mana regens slower while casting spells or in combat, so it's even less than that.
All in all, spirit isn't THAT helpful. It will lessen downtime a bit, but don't worry about getting just spirit. There's some gear that just has +spirit and no other stats. Personally, from the tests I've done, the gear's pretty crappy then in my opinion. But you're welcome to perform the tests yourself. :)