I have had very little exposure to Michael Gambon, despite being a television aficionado of many things 1955+. First thing I saw him in was Doctor Who-A Christmas Carol, where the character and therefore indirectly the actor, is perhaps difficult to like. However, Maigret demonstrates why he is first rate, because he is so likeable, you feel that you are "with him" with his facial mannerisms, his occasional yet succinct outburst of sarcasm, and his just scraping through to keep the bosses happy. We feel much in common with Maigret except for where he excels in a unique and slightly unorthodox manner of detection work. He plays the role in such a likeable way; he is on the side of those who report to him, firm but kind, rather than always adopting the superior position. In the same way he would arguably fraternise too closely with his inspectors, he likes some of his villains too much too which gives us a unique perspective we don't often see in other detective series.
I do not know much about the back history of Maigret, but nearly twenty years after this adaptation was made, I have thoroughly enjoyed these two 6-episode series wishing there was more. I would have to say that some episodes are better or more appealing than others, but that is true of any series, and opinions will vary as to the best episodes.
Granada, producers of the excellent Sherlock Holmes series, have shown their talent for crime series and made another significant contribution to both crime television and television in general. I will soon be watching The Singing Detective (bought when first came out on DVD but still not seen) as I want to watch more Michael Gambon.