Drupal 7 is ready for big sites? I know to right now Drupal 7 is stable since almost a year but the truth is Drupal needs third party modules for satisfy the requirements of almost any site 

Yesterday i tried jump to drupal 7 but the first thing to i see is to many of modules are betas, releases candidates and a lot  haven't a version for D7.

and many of this modules are essentials, modules like Ctools, panels,  references,  context, all of these modules are pretty useful and none of them have a stable version for drupal 7

So my question is,  Drupal 7 is  ready for a bigger site than a blog?  how many of you use drupal 7 with unstable modules for a client's sites?  

How risky is use non stable modules in drupal 7?
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!