Stefan--Welcome to the forum! Good question as well.
I'm very happy with the Tri-Elmar. It is compact and lightweight. Since it offers three different focal lengths, using it means you'll be changing lenses less frequently and reducing the likelihood that dust will enter the body.
If you want to be technical, it would probably rate as 'comparable to' lenses of the same focal length, perhaps not quite 'as good as' the others. (This is only theoretical; I don't see any difference.)
As David implied, even if it isn't up to the level of a 28 or a 35 or a 50 from Leica, it's still going to be ahead of the competition. S Reid's review was quite positive on the lens. It is definitely no 'slouch.'
If you need more speed, you might make another choice; but for me, I foresee that the Tri-Elmar will be my main lens on the M8 I'm waiting for, just as it has been my main lens on my M6.
Unlike Japanese brands, Leica offers only top-quality optics. If a particular lens appeals to you, don't hesitate.
--HC