As others have pointed out, this is basically the exact same story as Pastel, just condensed down to fit into 4 volumes. It seems like the author liked this story so much that he decided to expand upon it later on, as the ideas, themes, and characters and situations are almost carbon copies.
That's not to say this is bad though. Far from it, it's a very enjoyable story with fun characters, decent (although not great) artwork, and a really nice ending that leaves you with a smile on your face.
And in some ways, it's more enjoyable than it's successor simply because the story is a lot tighter, meaning you're not given a lot of extraneous chapters. This one deals exclusively with the two leads, and any other back story we may get is only in relation to how it affects their relationship.
However, while this does have a lot of benefits, I personally kind of prefer the slower pace of Pastel, in which the characters are given more time to grow and develop, and we get a lot more time to know and care about both the leads, and the accompanying players.
Also, like I said, while the artwork is good in this, if you compare it to Pastel you'll see that it still hadn't fully matured at this point, and is still somewhat rough around the edges, especially in regards to drawing the character's facial reactions.
Still, all in all it's a good entry into the romantic-comedy genre. While comparisons to it's more famous sibling are inevitable, that's still no reason to ignore this one, although I do think Parallel is better appreciated if read before Pastel. It's still definitely worth a look if you at all like the genre.