This is a wonderful topic. Opinions vary considerably. The following (from my personal FAQ) is more of a history lesson than a definitive answer to the question:
If you look at 61.57(c) (instrument currency) you'll see that the 6 instrument approaches that have to have been done in the prior 6 months must be "performed and logged under actual or simulated instrument conditions..." Some of the other requirements have changed through the years, but this one has been with us for a while.
Sounds pretty simple, doesn't it? Except some idiot thought to ask, "How much actual is actual?" What if you pass through a single scattered cloud on the way down for a total of 5 seconds of "actual"? Can you count the approach?
Sometime in 1989 or 1990, it seems FAAviation News ran an article that said that you had to fly the approach to minimums in IMC in order for it to count. Someone wrote in pointing out the illogic of a rule that meant that a very experienced pilot who flew hard IMC all the time would probably not be able to log the approaches, since most approaches don't involve breaking out at minimums.
In the July/August 1990 issue, FAAAviation News replied to the writer:
"Once you have been cleared for and have initiated an approach in IMC, you may log that approach for instrument currency, regardless of the altitude at which you break out of the clouds"
Problem is that this answer doesn't work either. Now, you're on a feeder route to the IAF above the cloud deck when you're cleared for the approach. You fly the full approach, enter the clouds just above glideslope intercept and break out at 200 AGL with 1/4 mile visibility. Oops! Sorry! You were not "cleared for and have initiated an approach in IMC".
(You're starting to see why I called the person who asked the "How much" question for the first time an idiot.)
In 1992, the FAA legal counsel chimed in:
"Second, you questioned how low a pilot must descend (i.e., minimum descent altitude or decision height or full stop landing) on the six instrument approaches he must log to meet the recent IFR experience requirements specified in FAR Section 61.57(e)(1)(i) (14 CFR Sec. 61.57 (e)(1)(i)). You also asked if an instrument approach "counts" if only part of the approach is conducted in actual IFR conditions. Section 61.57(e)(1)(i) states that:
No pilot may act as pilot in command under IFR, nor in weather conditions less than the minimums prescribed for VFR, unless he has, within the past 6 calendar months - (i) In the case of an aircraft other than a glider, logged at least 6 hours of instrument time under actual or simulated IFR conditions, at least 3 of which were in flight in the category of aircraft involved, including at least six instrument approaches, or passed an instrument competency check in the category of aircraft involved.
For currency purposes, an instrument approach under Section 61.57(e) (1)(i) may be flown in either actual or simulated IFR conditions. Further, unless the instrument approach procedure must be abandoned for safety reasons, we believe the pilot must follow the instrument approach procedure to minimum descent altitude or decision height."
Uh-oh! If you take the opinion at faces value, there's that reasoning again that essentially says that if you don't go missed, you can't log it.
There is a strong school of thought out there that says that what it "looks like" the FAA Counsel said is not what they meant. Note that despite the question, although the answer says that you have to follow the procedure all the way (unless it's not safe), it does not say that you have to follow the procedure all the way "in actual IFR conditions."
(You can see where his is much better fodder for arguments than anything else in the logging arena.)
The camp that says that the legal counsel didn't mean all the way in IMC (call them the "Rule of Reason" school) are essentially saying that "How much" is one of those undefined terms. Not everthing is susceptible to precise definition. Try to thing of all of the scenarios and come out with a rule that covers every probably (let alone possible) approach scenario. How many pages did you use?
When Part 61 was revised in 1997, there was a proposal to write the rule so that, in order to count, approaches had to be flown to MDA or DA to count. They got a lot of comments, including one that said,
"One commenter suggests revising the definition to permit the pilot to terminate the approach prior to DH or MDA for safety reasons. Another commenter proposes to define "instrument approach" as " * * * an approach procedure defined in part 97 and conducted in accordance with that procedure or as directed by ATC to a point beyond an initial approach fix defined for that procedure." The commenter explains that this definition would allow for logging instrument approaches that require some portion of the published approach procedure to be followed in order for the pilot to establish visual references to the runway"
The FAA decided against the new requirement.
Some point to the fact that the FAA posted this comment as support for the rule of reason approach.
Whew!