I love the S12 platform. I've owned/own a bunch of them. My go to shotgun is a S12. I shoot it more than any of my other guns. First I advice you take any definitive statement about reliability, the need to break them in, quality, etc with a grain of salt. These guns are not built with super tight tolerances or with the greatest of quality control. I've seen a bunch that will run fine with lighter bird shot right out of the box and I've seen others that wouldn't even after 250 or what ever people want to say the magic number of rounds is. I've seen S12s with canted sights, and a bunch more that were fine. I've seen varying quality of finishes. Simply put they vary.
Honestly I think I would take a number of tube fed guns over a stock S12 out of the box. You need to spend money immediately on an S12 IMO. First and most obvious is the fact the gun only comes with one 5 round mag. If you only have one 5 round box mag I'd rather have a tube gun I can top off. This is obviously easily fixed by buying more mags. As stated you will have 922r problems if you get mags that hold more rounds than that without modifying the gun by removing another countable foreign part. Another consideration is that mags aren't particularly cheap.
To me there are bunch of things I don't like about the stock gun that are pretty easily remedied but it costs money to do it.
The stock gun has bad balance and bad ergos. Reloads on a closed bolt are a difficult and from what I have seen in real life present shooters more problems that what most on the internet claim. At a minimum they take practice to get down. The stock trigger is crappy.
I've always said that I think the S12 is a crappy $500 shotgun but it can make a pretty sweet $1K shotgun.
Here is a thread of one of my project saigas. In it I think there are some of my musings about the stock S12 and why I modiffied things. Maybe it will be of interest to you.
http://www.thehighroad.org/showthread.php?t=
It should perhaps be noted as well that Guabe Suarez ranks even the stock S12 very highly. I don't recall exactly but I think it was top five on his list of fighting shotguns. I could mistaken about that and I'm sure someone will set the record straight if I am.