I've used Metz for a number of years now and have no complaints about the flash units themselves although I have been less impressed by some of their battery packs.
The use of the flash depends on 1,001 different variables so there are even less hard and fast rules than for using a camera. In fact due to the rather sensitive nature of digital to minor variations in light levels (film was so much more forgiving) flash has to be used with a certain amount of caution. I must admit that I have never successfully used auto or TTL but instead rely on the histogram, gut instinct and more than a bit of PS to see me through. Highlighting is perhaps the most difficult situation and here bracketing is invaluable but it is still very difficult to get a reasonable effect especially in dull lighting where the mismatch in lighting levels between subject and background can be very pronounced with only the lightest touch of flash.
Where I have enjoyed some success is in the theatre or similar situation where there is reasonable distance between you and the subject. Here I just whack the unit on full or half power and set the camera to match the flash, unsophisticated but it can work well, these were taken with a K10 using this approach -
Pallas Players
On the other hand I have struggled with cheque presentations in pubs etc where there is a lack of room and decent background and even the higher ISO's of the K5 haven't helped, no doubt more practice and experimentation will though but there are no easy answers.