You know how these rote greetings work—most of the time when we ask someone how it's going with them, we have as little interest in hearing the answer as they have in giving one. Admittedly, when I say "how's life?" to someone at work or to a passing acquaintance, it's usually spoken as much as an incurious courtesy as "how's it going?", but it has a greater likelihood of surprising me by eliciting responses that aren't worn into automation. Even if the person simply answers "good," there's usually still a half-second during which they actually process the question and consider their answer (or consider whether to consider their answer).
There's one response that gets my goat: "beautiful." Especially when it terminates a recapitulation of the question. "Life is...beautiful."
It's usually the damn kids who say it—the damn kids and the double-damned buzzed single thirtysomethings. And they usually say it like they're epiphanizing on the spot or imparting some profound truth.
It sounds like a premature conclusion, especially when it's coming from the mouth of an undergraduate on spring break or a late twentysomething enjoying their second gap year. It has a tinny ring to it; it strikes me as the evaluation of someone who's visited life from time to time, but has never strayed far from the touristy parts.
To these people I have a few friendly suggestions toward a more multidimensional conception of life so that they may more accurately appraise it:
Outlive your child; have a stillborn child. Live your whole life enjoying the use of your limbs, and then break your neck and live as a paraplegic; or otherwise suffer a debilitating injury that renders you incapable of doing what you're most passionate about. Lose your home, your pets, and every material object you own in a hurricane, an earthquake, or a freak accident when a train carrying crude oil jumps the track. Suffer from an incurable chronic illness that slowly kills you and puts you in constant pain that only gets worse. Believe in someone and watch them let you down. Repeatedly. Believe in something and watch it betray you, watch it fail. Repeatedly. Fail at something yourself—try something, believe in it, stake everything on your success with it, and then fail utterly. Then try something else and fail at that, too. Watch people—more than one person, two, three, ten twenty—who can be objectively judged as unscrupulous, mean, and generally awful human beings, watch them enjoying more success and happiness in their lives than you. Struggle with addiction for a few years. Get raped. Have a traumatic experience that you never get over completely. Happen to be at the bus station, the mall, or the college campus when the pipe bombs go off and the armed zealots or psychopaths start indiscriminately shooting people. Get in a really bad, really stupid fight with your significant other and then lose them in a car accident before you have a chance to reconcile with them. Suffer third degree burns all over your body and live the rest of your life with hideous inoperable facial scars. Get cancer, beat cancer, and then get another kind of cancer. Be lonely for a long time. Be constrained to work a job that you hate, that exhausts you physically, mentally, and spiritually, for many years. Get beaten up for no reason. Get beaten up by a cop. Get your ass kicked by a group of belligerent motherfuckers in a public place and watch people hurry past and pretend not to see you as you bleed and scream for help. Live in a war zone for a few months or years. Live in a place where people are regularly dying from starvation. Stand trial for a crime you didn't commit; go to jail for it. Witness something awful happening to someone you love, something you might have personally prevented. Do something really fucked up, something you never imagined you'd do, and get away with it, and live with yourself knowing you really are the kind of person who'd do what you did.
If you've had life shit and vomit on you and you can still sincerely say it's beautiful, then wow—that really does mean something. And you're probably right.
In the Zen tradition, the mountains and rivers really are nothing more or less than mountains and rivers, but proclaiming it means nothing unless you've taken the trip and returned again. Life might be beautiful, but it's fatuous to announce such a verdict when you don't know it well enough to understand how poisonously ugly it can be and can transcend it.
Until you're there, I think the most appropriate answer to the original question is "it has its moments." Or treat it like a weather report: "sunny today; chance of rain tomorrow."