With a multi faceted answer at that!
If you can make your own GREEN juice- that is ALL GREEN VEG, then yes- help yourself! It's a great way to get loads of vitamins & minerals from fresh, raw veggies. You can throw in some raw garlic for all of its goodness, some ginger to aid in digestion or a pinch of turmeric for its anti inflammatory properties. I wouldn't say you should rely on this as your ONLY source of green veg, but as PART of the rest of your diet, it's a great idea. Make sure to balance out the juice with some healthy protein (you could mix in some egg white protein powder, or have some sliced turkey on the side...) and a good dose of fat- like some freshly ground flaxseed.
On the other hand- if we're talking about many of the juices on the market, no matter how 'natural' they may be, we need to be really savvy about reading labels. Some are, in fact 100% juice, but really high in sugar, and as such, should definitely be portioned out and eaten WITH other foods to bring down the high glycemic load. Others have soy or whey protein added, and I'm still only getting started with those that are sold in reputable establishments, like Whole Foods!
Still others may not even have juice in them, or just a trace of juice listed on the ingredient panel after corn syrup ( don't get me started on that one!) and before the red dye # 40!
Juice bars also can be hit or miss for the same reasons. Fortunately many offer ingredients of all their juices and smoothies to the public so you can choose your best option, or perhaps make one up, just as you do when you're ordering Paleo off a non-paleo menu.
I enjoyed a lovely juice today from the local Co-op market- cucumber, spinach, celery, kale, lemon and garlic-which I found delicious and quite invigorating...much better option than a trip to the local cafe for a whipped up mocha!