How to use a hand light-meter?

I posted this question in the darkroom discussion group, but was told it was not the appropriate group to post, so I will try here in the film & labs section instead.
Film needs to be exposed properly for the best results, so I hope nobody here takes umbrage.

As obtaining correct film exposure is of fundamental importance to provide good negatives, would a hand meter be a worth while investment instead of relying on the TTL metering of my 35mm SLR Camera that I use at the moment?
I also use this to transfer readings across to my Mamiya RB67 medium format camera.
What are the advantages and disadvantages of incident versus reflected measurements?
I would like to know more about using a hand meter before parting with my money.
I saw a Kenko KFM-2100 on-line, but it can be any other models and brands you can think of, as I am not sure what to buy. Please help and advise.