Compound Master does work (according to *******), however it simply doesn't work how you expect it to work ... It says it adds 65% to your compounding ability, however according to him it works, and it simply adds 65% to the formula of being able to compound the numbers listed on the item.
Compound Master is misleading, but it's FOR SURE, that it does NOT add 65% to compounding result (as in if there is for example a coumpound item that is 100-200, the result will NOT be 165-330, it'll simply increase your ability to get a slightly higher amount within the 100-200 range than what you would have gotten without it. So say that you would have gotten 125 without Compound Master, with compound master you'd have gotten 141.. maybe..)
Basically from what I understand of Neo****'s explaination of it is that it increases the result by 65% (but I can't be sure of that.) So with 125... it's 25% of the maximum you coudl get so it's 25*1.65 = 41 Something like that ...
Like say ... the item is 5 - 15 for DA, and with your "roll" when you compound comes out to 50 (Imagine a roll of a 100 sided die, each # representing a percent.) which would result in you getting a +5 on that compound, which would be +10 DA. Now with compound mastery that 50 would be multiplied by 1.65 and come out to be 82.5, which would increase your compound to +8 (because it's rounded down.) which will be +13 DA. However the more than you end up with LOW rolls, the less you notice an effect on your compounds. Say your roll comes out to be 27 (it'll end up being rounded down) so +2, that'll be a 7 result. With compound master it'll be 27*1.65 = 44.55, which will end up being +4 or +9 DA total.
Considering Nate in the recent months however your roll will most likely be around 15 -.- 15*1.65=24.75 which = a simple +2 result, meaning a 7 DA compound... And on lower items with a lower range between results you'll find it being much worse..
For example... Level 50ish or so compound items that are like 2-6 result... 1-20 roll = 2, 21-40 roll = 3, 41-60 roll = 4, 61-80 roll = 5, 81-100 roll = 6. (Note that you will not go over the max even if you roll a 100*1.65, max is still 100.)
So really.. Compound Mastery, if you even choose to get it, should be gotten at *higher* level when you've got plenty of TM to spare and where it won't get in the way of the TM you need to spend on other skills for leveling.. However some months ago there was lots of people resulting in getting minimums all the time with Compound Mastery trained, that is where the whole thing started about it not working at all, and even hurting you :X
By the way, there is differences between Item Detector and Basic Detect aside from the depth... First thing is that Item Detector is only a 3x3 space (maximum of 9 items), with a VERY long recharge time (15 seconds.), Also... even when Item Detector is mastered, it FAILS quite often -.- So this is why I reset Item Detector down to level 1 only recently. (Note that Item Detector can detect at maximum depth even at level 1 skill because Depth with ID is based off your DA, not skill level.) Basic Detection however is spammable (1-2 second recharge time, and rarely fails.), and can reveal up to 7x7 area at 110 depth, Maximum of 50 items found... (The max depth you'll be digging in later areas is 200 and 250, which is where Item Detector comes in to play, BUT Basic Detection can still find items that are 110m and below in areas of depth that are max depth of 150 or so.. And you really won't touch 200 Depth often until level 120+ probably because of the rarity of drills used to dig that high (unless you only use Myshop drills.) and the monsters in those areas where you'd be digging that deep.)
Overall:
Compound Mastery = A gamble of TM points ...
Item Detection = Often fails even when Mastered, Long Recharge, Level 1 can still find max depth, and only 9 items every 15s. (15s, if it doesn't fail, which it will, often

)
Basic Detection = Can only go to 110 Depth, but can find up to 50 items at once, every couple seconds, and gives you a sweet view of patterns of items in cluttered areas, also SUPER for Events that involve Drilling.