With dd1 we used OM for kinder, have been using Moving Beyond the Page for first, and are strongly considering going back to OM for second. My pros and cons are a bit jumbled up, so bear with me!
DD1 attends a 2 day/week homeschool enrichment program that is strongly Waldorf influenced but not "100% Waldorf". Since OM is also not entirely Waldorf, they mesh pretty nicely. We've had some dissonance this year using MBTP and I know that using OM at home would resolve that. There are a few articles on the OM home page that discuss how OM varies from traditional Waldorf that make for interesting reading (in a nutshell, OM removed references to the various saints and christian holidays found in Waldorf, OM is more in line with state/national standards so includes reading/math from the start, OM doesn't use the original Grimm's fairy tales, and OM avoids some of the more "philosophical" elements of Waldorf education).
DD1 is very focused on arts/crafts and the OM curriculum works arts/crafts into just about everything. However, dd1 found the Kinder crafts to be pretty simple (make a pinwheel, make a crayon leaf, paint with colored water on snow), and we ended up purchasing the First Grade Nature Craft book to. I guess that the art focus of OM is a pro, but the match between child and "grade level" might be off for some kiddos. You can purchase the individual bits (either from OM or a second hand vendor) so it's not a big deal, but it was a bit frustrating.
We are a very academically oriented family. Dh and I, and most of our relatives and friends, have multiple advanced degrees and feel that a strong "traditional" academic foundation is crucial. OM is a bit more "relaxed" than other programs we explored, but it does meet national standards and their middle/high school program is kick-butt. With a babe due more or less the first week of Sept and two kiddos to homeschool/get to enrichment classes I am not about to complain that the OM program is a bit "gentle" in terms of the amount covered in a day/week/month/season but it's right on the edge of what I consider "enough" for the early grades. I think being on board with the basic Waldorf philosophy of grow/slow is important... OM does move more quickly than traditional Waldorf, but I'd guess most kinder kiddos already know their alphabet and numbers, the names of primary colors and shapes, and things like that. I found the Kinder program to be a nice "review" and it encouraged dd1 (and I) to explore some concepts in more depth, but she didn't "learn" anything new. Eventually dd1 got bored and, after reviewing a friend's 1st grade OM curric and finding that dd1 already "knew" that material too we went with MBTP for 1st grade to avoid further "boredom".
From the parent side, the OM teacher/parent guides are NOT as detailed as those provided by Calvert or Sonlight (for example). Instead of breaking things down by day and showing you what will happen that day/week, the guides set the entire week out at once and leave it to you to decide what will happen when. Again, I appreciate the flexibility but kind of wish there was a little more structure. Also, you need to plan ahead for nearly everything. Check out the sample on the homepage and you'll see what I mean... for instance, the "this week, make bread with your child and shape the dough into letters" bit. Yes, there is a recipe in the craft book but there is a lot of prep/time "hidden" in that one sentence! For the kinder year it wasn't a big deal... I'd just skip what I didn't have supplies for... but it was frustrating. It's actually one of my main cons and the reason I'm not 100% sold on using OM this coming year.
Price is another factor. While not as expensive as some, it's a fairly big chunk of change for the Kinder year. The craft packages are handy but you'll probably do better locally in terms of supplies. There are lots of second hand options (and the big spring sale!) since the curriculum doesn't have any consumables so to speak... everything is done in lesson plan notebooks or kept in files.
We're a pagan family and found the religious neutrality of OM to be a big plus. Traditional Waldorf is "too christian" for us but we didn't have a problem with OM.
We don't really like OM's math... the stories are cute, but the Kinder pace was very slow (for a child who already knows the number 2 a whole week of "is this 2?" gets to be a bit much). We prefer RightStart math from the MBTP program and RightStart actually suggests starting in kinder... I know dd2 loves it.. Anyway, a child who is less interested in "narrative" and more interested in "numbers" might get frustrated by OM's (or Waldorf's) style of math. However, I do like that OM introduces math from the start, and like the way it ties numbers into the lived environment, and a child who doesn't like numbers might really enjoy the OM approach.
Sexism/Classism/other -isms: it's more a function of Waldorf than OM, but it can be annoying. From the anthropological side, Waldorf as an educational philosophy is very much a product of it's time and due to the nature of the system, change is very slow (since the whole point is to retain the slower/heart-hand focused/collective spirit of an older time). OM has modernized in some ways, but in others the hetero-normative and gender-normative elements are built in. Every king has a queen, brave princes rescue beautiful princesses, girls care for babies and boys play at knights, and so on. OM has branched out to include more non-European source material and the older grades are more "modern" in their approach to certain issues, but...it's still an issue. I'm not sure if there is a "perfect" curriculum out there in terms of awareness (though Global Village may come close), but I think it's easy enough to correct for in the OM program since it tends to be more of the "obvious"/"oblivious" -isms rather than an intentional filter being applied to all aspects of the learning process.
Anyway, we liked OM well enough but think we'll like it more for 2nd grade than we did for kinder or would have for 1st. :)