I've had some experience with all three Microsoft games (that I know of) that one can earn achievements with while using an iPhone: Wordament, Sudoku, and Solitaire. I'll be mentioning the other two on occasion as I talk about Microsoft Sudoku, and this review will be pretty similar to what I had to say about Solitaire.
Sudoku is going to be sudoku no matter what happens: There is exactly one correct solution for every game. To break up the monotony, Microsoft included a warped variation of the game in the form of irregular shape puzzles. This simply chalks things up to forming nine weird shapes out of a regular sudoku puzzle, which in turn changes the strategies to a fair degree. But is this enough to keep things fresh? I don't believe it is.
There are three different game modes in Microsoft Sudoku: normal, irregular, and an ice-breaking game. The third is only available through daily challenges, which means it's essentially an extra to the other two modes. When the achievements ask you to play 500 games of non-challenge sudoku, you're going to get tired of the two types offered to you very quickly. Numerous other variations - twin, killer, etc. - could have been included to make things at least somewhat more engaging than the two we're allowed to play.
The three Microsoft phone games I know of are guilty of throwing ads at you every five minutes or so. While this is sort of the norm for free phone games, I would have definitely preferred to buy the game outright instead of having to wait a half-minute just to get back to playing another game or two of sudoku. Unlike Microsoft Solitaire, Microsoft Sudoku's premium service doesn't grant you daily challenge access, only the removal of ads. This only makes sense, as the challenges are randomized every time you play them.
Achievements aren't something I would rate a game on, but since this is TrueAchievements, this is more of a bonus category for me to talk about. Some achievements take far too long, which his normal for Microsoft phone games in regard to daily challenges, but I don't believe I'd want to play 500 games of non-challenge sudoku unless someone was paying me or I was masochistic.
Pros & Cons
+ Sudoku is fun. (Please pronounce it correctly.)
+ Allows for earning achievements (which is extremely rare for iPhone users)
+ Difficulty curve has been created well
- The only app that gives my phone screen burn-in, making it more of a danger to play
- Ads are a looming threat and are of a low variety, so you get sick of them very quickly
- The game becomes boring very quickly
? Some may find the completion requirements to be absurd
Conclusion & Summary
Microsoft Sudoku is definitely sudoku, and that's all it really is. The game feels more like a chore than anything else thanks to the extensive achievements and ad frequency/length. The variety is greatly lackluster and may even convince some that sudoku is too simple once they've figured out everything there is to learn. Only those dedicated to the puzzle game should bother downloading it.