If you've somehow managed to avoid playing Rainbow Six Siege despite its many sales and free weekends, you'll be glad to hear that you now have another chance. Simply visit Steam, click on that shiny green "Play Game" button, and you'll be able to explore what Rainbow Six Siege has to offer until Monday.
As for whether its actually worth playing, I would say the answer is a definite yes. It might not be my cup of tea, but even I can appreciate the fact that Ubisoft is constantly working on making Rainbow Six Siege better, both in terms of bug fixes and content updates. So if you're up for a tactical team-based shooter, and you don't mind a bit of a steeper difficulty curve, Rainbow Six Siege is well worth trying out, especially when you can do so for free!
If you find youself enjoying what you see and you decide to purchase it, I need to warn you that the cheapest Starter Edition is a massive trap. You'll save €10 initially, sure, but you'll also have to grind an unberable amount of in-game currency in order to afford the Operators you like. So trust me on this one, just go with the Standard Edition - keeping your sanity intact is worth the price.
And finally, I'll leave you with one of the original gameplay trailers, just for good measure. It's obviously a bit outdated at this point, but it'll still give you a decent idea of what to expect. Enjoy!