Is it easy to learn how to play?

It's more difficult than Dark Heresy or Rogue Trader. If you are an experienced GM, you'll be able to handle it just fine. If you're not a veteran, I recommend trying DH or RT first. Otoh, if you are a Space Marine fanboy or are willing to put a good deal of work into learning the rules, you can have a go at it.

Alex

As a player, all you really need to know is the core d100 mechanic, and for the most part, you are set.

Some people do have issues with the untrained/trained, basic/advanced skill differences, but they are intuitive enough it shouldn't be an issue. There are plenty of posts here answering that.

The big thing Deathwatch adds that does make it a bit more complicated are the Space Marine specific features (squad modes, marine implants, etc.).