I alternate using different styles. For most applications I just use 2 fingers, and only switch to 3 for fast runs, where the extra speed is needed. Here's my thoughts on different styles:
R=Ring M=Middle I=Index
RMIM
"The Wave" - I pretty much gave up on this style myself. I know Steve Bailey uses it, and Steve Digiorgio (awesome metal player) does too. With great results. The advantage I see to this technique is that when playing groups of 4 notes, you'll always start each phrase with the same finger. The downside is that the middle finger is being used twice for every 4 notes, so it's working harder.
The other downside (for me) is that you have to go both forwards and backwards with the finger 'roll', and for me going from middle back to ring is a very unnatural feeling. I used to spend hours practicing just playing even 8th notes alternating middle and ring fingers, and it never got much better. Alternating between index and ring or index and middle is fine, but not between ring and middle, for whatever reason.
Then there's RMI
This ones works great for triplets, or 16th 16th 8th patterns. Any 3 note patter, but for 4 note patterns it gets tough. Like its been said, you'll be starting each group of 4 notes on a different finger each time and getting not only the accents right, but just making fluidly connect with other rythms/picking patterns would take a LOT of work.
What I've taken to doing is a combination. A lot times I'll do RMIMIRMIMI, and continue with that pattern. Or if I'm moving across strings I'll use rakes along with a 3-finger pattern to get a faster run. Something like this:
-----------------------4---------------------------
-------------4--5--7-----7--5--4--------------------
-----------------------------------7--5--7-------------
--------------------------------------------0-----------
__________R_M_I__R_R_M__I_I_M__I_I______
I do some patterns like that in here (at 0:51):
http://www.youtube.com/watch?v=lMGhJ9JAECs
But for straight 16th notes runs, one note, one string kinda playing I've been doing something...different. I go Ring Middle Index, and then for the 4th note I bring my index finger back up to hit it from underneath the string. It keeps the nice flow of RMI, which I've always been very comfortable with, while adding that 4th note. You have to work at getting the index finger to come back up with the same force, but it's worked very well for me so far. The tone is a little different (fingernail hitting the string), but not enough to make it sound completely out of place. And when you're playing that fast, nobody is really paying much attention to your 'tone' anyways
I'll also do that with just 2 fingers (MII[up]MII[up]...) to get the same triplet pattern that RMI gives you, but with a different feel.
Then you can combine that all to come up with something like this:
-12-13-15-15-15-15-13---12----12------------------------------
--------------------------------15----15-13-12-15---13-12------
---------------------------------------------------------------15--
--------------------------------------------------------------------
_M_I_I[up] R_M__I_I[up]_M_M_R__R_M__I_I[up]_M__I _I_
^That took a long time to think about and write out. I hardly even think about it when I'm playing anymore. So hopfully that makes some sense, in a nonsensical way. Have fun
