I've owned a Moleskine pocket (ruled) notebook for over five years now. I've only filled half of it, but I'm constantly amazed by the amount of punishment one of these notebooks can take. I always use a B or 2B mechanical pencil to jot down notes, and even my earliest entries are still legible.
I needed a proper journal for technical use, and was a little hesitant at choosing between the squared and the plain A4 notebook. I eventually purchased the squared, still slightly fearing the thickness and darkness of the gridlines. When I draw, I don't apply too much pressure, and I was afraid that the gridlines will dominate every entry.
Well, my fears were mostly unfounded. The lines are precisely the same thickness and color as the ruled pocket notebooks. And so is the paper. This does mean that I have to make more of an impression when I draw (as opposed to writing), but it's not a dealbreaker.
If you own a pocket Moleskine, this is the big brother in every respect (down to the little historical insert). It is quite hefty, despite its thin profile, which is likely due to the heavier paper used in Moleskines. You don't notice the weight with the pocket notebooks, but it definitely shows in the A4s. I tend to treat the journal a little more gingerly due to its weight (like never grabbing it by the cover alone, like I freely do with the pocket notebook).
Overall, a good buy if you apply a little bit of pressure with pencils when you write/draw, or if you use a pen. If you like feather-light pencil touches to register, you'll probably want the plain Moleskine A4.