Drupal 7 is ready for big sites?
Q: &quot;How risky is use non stable modules in Drupal 7?&quot;
A: &quot;Depends.&quot;

Without wanting to be obtuse, in late 2011 this is a question only you can answer, based on your abilities, needs and experience.
The simple answer is: Betas are usually fine, Alphas should usually be avoided, RCs are almost certainly OK.
If you're happy to hit a bug and help fix it or apply patches, you'll be mostly fine and able to really help the Drupal community. If not, the risk for you is higher and could become a show-stopper.
If you're using one of the 'biggies' with tens of thousands of users (CTools, Panels, Views etc) you can probably use alphas/betas without hitting many major issues -- or you'll be one of thousands with the same problem which means it'll get fixed quicker... But if you're using less common (but still complex) modules, or it's critical you have complete reliability in your site, then you'll need to think twice - and maybe stick with D6 for a while.
A safer approach if you're worried and don't have the expertise to get your hands dirty in code or patches is probably: use Drupal 6 but choose your modules carefully so there's always a D7 upgrade path. Upgrade when you're happy it's right for you.
Ultimately you might have to use Drupal 7 because it provides certain features you cannot get in D6.
To close I would say that, for the most part, Drupal 7 and its ecosystem of modules is ready for most people/sites/projects. But things are changing by the day, so if you don't feel comfortable now you might in do a month or two.
Do your research and if you have doubts or hit snags, Drupal 6 is generally still an excellent choice and will be for many more months.
Good luck!