"The Devil is a Part-Timer!" (hereafter TDiaPT) makes for pretty good entertainment. I do have a few complaints, which will come later, but generally the positive aspects of TDiaPT outweigh the negatives, and most of my complaints are conditional.
The story is lacking in some respects, but this is by no means a serious series, no matter what the first five minutes lead you to think. TDiaPT made short work or making me laugh, and continued to do so for almost the entirety of the season. Some of the people I've talked to concerning TDiaPT expressed concerns about how little time the series spends on backstory, specifically the Ente Isla conflict, but objectively I believe this has little impact on the quality of the show. With what we're given, I was able to get the jist of what happened before Satan showed up in Japan. The show does an outstanding job of blending drama and slice-of-life style, while maintaining regular comic relief.
As far as art style is concerned, I found the show very pleasing to watch. The action (when it does appear) is fast-paced and fluid, and the calmer scenes feel just that, calm. The character design is above average, with powered-up Satan, Emi and company looking quite believable as dangerous fighters, and human-form aforementioned looking like average folk.
The soundtrack doesn't play as big a role in this series as it would in a more dramatic example, but when it is needed it delivers. For the sake of ease, I'll throw subtitle concerns in this category as well. The people of Ente Isla speak a different language (in both dubs) than the people of Earth. The language is essentially English with a few letters switched about, but the medium through which I watched the show didn't provide any subtitles for this alternative language. Downloaded versions may come with subtitles, and if you're already watching subbed this shouldn't become an issue.
The character developement is surprisingly good, for what TDiaPT is. Without spoiling much, I can tell you that Satan and friends will act quite differently from what you might expect.
For enjoyment, I found TDiaPT to be a great palate cleanser to watch between more serious series. The story is generally jovial with few exceptions.
Overall, this is a good show to check out if you're getting burnt out on mature, intense stuff.
