Yes it is a strange one. Moyes did well at Preston, almost getting them in the PL. When he came to Everton in 2002, he reinvigorated the place and was like a massive breath of fresh air. The whole attitude became more positive and determined and we escaped relegation, thanks mainly to his arrival (IMHO). The following season was the best for several years, and apart from a couple of blips (2003/04 when we came 17th and 2005/06 when we had a disastrous start but recovered to finish 11th), we always finished in the top 6 or 7 and even qualified for the CL one season. Many, including me, felt that with more money, he would have taken us further and threatened the established mega rich CL clubs more often. He was a pragmatic manager for us, and whilst at times that resulted in some dour football, at other times we played some great stuff.
However, I reckon Moyes stayed at Everton too long for his own good (but to our advantage because he kept us competing above our financial weight). Because of the financial restraints, he became very cautious and even negative, especially when playing the big sides and ultimately he would always bottle the big occasions such as the 2009 cup final and 2012 semi. The amateur shrink in me reckons circumstances caused him to lose his early hunger and drive, and conditioned him into a negative, cautious mindset which was reflected by remarks he made such as "taking a knife to a gunfight" when we played Man City one time. That mindset seems to transmit to his teams now,
He probably is judged a bit harshly for his time at Utd, apparently his results weren't any worse than Van Gaal overall, but since then at Sociedad and Sunderland he has struggled badly. Even considering the seemingly chaotic boardroom at Sunderland and the lack of player quality, he seems to have lost the plot (and maybe the dressing room), and reports suggest the fans are turning on him big time now.
Very sad, but I think he is yesterday's man, he might not have been if he had gone to somewhere like Utd after about 5 or 6 years at Everton, it might well have been a different story then, The irony is that Everton needed him much more than he needed Everton, and now he has gone, Everton's financial situation has improved massively. I hope he turns things round for himself and Sunderland if he stays there.