It seems that in Beijing, you have a choice between low-range and low-quality (Yaxiu, Silk Market, ... ) and high-end, high quality, but quite expensive (Senli, Dave), with very few options in the middle class. Value for money does seem to be on a linear scale here. You see the same thing with neckties and other accessories: either you pay Y30 (never more!) at Silk Market, or Y1000 at Oriental Plaza. I did not find a nice place to buy middle-class neckties in Beijing yet. That said, I have not yet tried Carol and Luc, but have heard some good things about them.

However, I did order a three-piece suit from Senli last year and it was amazing. Slightly on the expensive side (Y5000, but a three-piece is essentially one extra piece of clothing, and I did choose nice fabric), but absolutely worth it, and I'm not being paid to advertise. On the other hand, I also had a suit done at Ruifuxiang, the big silk store on Dashilar, and the cuff buttons started falling of after about a month or so. Quality is being skimped on in the most ridiculous ways here. The tailoring itself was not bad, but details like buttons, etc., very often just are badly done. And, of course, there is the problem of fake fabric (discussed in another thread, I believe).

In the end, the question would be what you need the suit/shirt for. If you're a student, and you need it for the odd function, cheaper will be just fine. For business, however, I wouldn't recommend it.

On a last note: it may not exactly be tailoring, but Gieves & Hawkes have a shop in Yansha Outlets on the 4th Ring Road with made-to-measure suits, starting from about Y2000, if I remember correctly.

3.3 and yashow area sucks. They all use poly blends, no straight wool, I have tested it. The fitting is ok in all of them, but the service is bad at Lisa's tailor shop.

If its just a basic alteration, any tailor is up for it here. But unless its drastic you should only pay around 10-30 rmb.