[I know this shows 0 hours played, but my achievements should prove that I'd previously bought and beaten this game. In fact, I played it three or four times over!]
Hexcells Plus is everything I am always looking for in a puzzle game, distilled to a deceptively simple interface. Though comparisons to Minesweeper are not unwarranted, these comparisons miss a crucial difference: in Minesweeper, the final moves often (and annoyingly) come down to chance, debasing all the carefully planned moves you'd made before should you--at the end of a long game--happen to click a mine.
In Hexcells (and Hexcells Plus), however, the game gives you JUST enough information to never, ever have to guess. But you will often spend many, many minutes searching for and debating your next move. While the game provides various clues as to where you can find Hexes and numbers, it does not (at least past the first few levels) make this obvious (as it is in a game like Picross, for example, where every column's number of squares is provided). It only provides clues for a limited number of columns, rows, sections, etc.; based on this, and based on tricks you'll pick up and teach yourself as you go along, you'll have to very carefully make your next move.
And this is the crucial difference: any mistake you make in Hexcells is your own fault--thus Hexcells corrects an unfortunate norm in the puzzle genre. So, between chess-like demands on your every move, between the security of knowing the game will not force you to guess, between the pleasing visuals and soothing ambient soundtrack, Hexcells Plus is one of, if not THE, best puzzle games I have ever played, and I play many.