Add to that the fact that your talent choices can increase your spellpower and your regen, and a math-phobic like me turns to jelly when I try to consider any intricacies involving int, spirit, mp5. I just want to know those magic numbers at the end, I don't really want to try to wrap my brain around how they are calculated.
So I finally bothered to get RatingBuster. RatingBuster does all the complicated equations for you, taking into consideration your spec, existing gear and innate abilities, and spits out a tidy little stat summary, colour-coded for your convenience.
It will tell you whether you are gaining stats (green), losing them (red), and also tells you things like how much crit you gain from intellect on an item, how much regen you gain from spirit on an item, etc. As you can see in the above picture, every stat is an improvement on my old gloves (all of the numbers on the right are green), and it even tells me how much healing I would gain from spirit (since I was in ToL when I took this screenshot).
It will even re-calculate these numbers as you change your gear around. For example, here is a trinket with 184 spirit (on use) on it. With no gear on, it gives me about 20mp5 (for 20 seconds every 2 minutes). With all of my gear on, the mod takes into account my mana pool and my full stats, and the 184 spirit now gives me around 50mp5.
So you can see that as you change items of gear, the mod will recalculate the stats that you get from your items (and buffs). A trinket with 184 spirit might give you 48mp5 for 20 seconds, but when you pick up a new pair of pants, that same spirit now gives you 50mp5 for 20 seconds. There's still the brain-exploding task of comparing it to other trinkets (hate), but at least it can give you an idea of what 184 spirit will mean in terms of actual regen.
And look at this: it knows that spirit will give me extra healing while I'm in tree form. In caster form, it shows no healing bonus from the spirit, but in tree form, it calculates it for me:
It's really handy for times when something drops and you need to decide in a hurry whether it is good for you or not. Of course, with the consolidation of healing and damage into spellpower, you do still need to be careful about taking items that are probably more geared towards casters, but this mod will at least help you to know whether something is better than your current gear.