Is Romeo really in love?

The story of Romeo & Juliet is well known and often seen as a romance. It even begins with Romeo professing his love to Rosaline in the first Act. However, because Romeo and Juliet ends with a tragic death of the two main characters, it's easy to see why this play is classified as a tragedy despite the romantic scenes throughout the play. Initially, Romeo is said to be in love with Rosaline. He speaks highly of her and his desire for her. It wasn't until Romeo observes Juliet that we see true love developing between the two.
Romeo put greater effort with pursuing Juliet. An effort that is never witnessed by his short interaction with Rosaline at the dance. Romeo is persistent in courting Juliet, he comes to her balcony and seeks marriage despite their families' differences. As the play ends Romeo's love for Juliet can not be doubted because he gave his life to be in death with Juliet forever. He could have chosen life and Juliet would never know. Yet, Romeo is a man of his word and believed living without his love, Juliet, would be miserable.

Shakespeare was intentionally ambiguous in presenting Romeo's love throughout the play. He is introduced as gentle and heartsick in the opening scene, preparing us to accept him as a loving character. Further, by the end of the play we may be convinced that he feels love deeply, given his passionate fury at Mercuito's death to Tybalt in Act III. However, his sadness flees the moment he sets eyes on Juliet suggesting that he may be afflicted by lust rather than true love.
When Romeo expresses his love to Juliet at her balcony (Act II Scene I) we are left in no doubt about his feelings as he compares her variously to the sun, the stars and an angel in heaven. This language could invite some scepticism given his apparent heartfelt sadness at not having his feelings reciprocated by Rosaline earlier, but if we compare his language in the opening scene to the Act II Scene I we can see that he is far more joyful and hopeful (e.g. 'Ay me, sad hours seem long' becomes 'She speaks: oh speak again bright angel!"), possibly a sign that his feelings for Juliet are truer and deeper than his previous infatuation.
His revenge on Tybalt and his suicide when he believes Juliet to be dead are sure signs this his feelings of love (for Mercutio and Juliet respectively) are genuine and run deep. While we could easily interpret both actions as being suggestive of thoughtlessness on Romeo's part and therefore shallowness of character, his language in the final scene is again compelling in arguing his genuine love "I still will stay with thee; / And never from this palace of dim night / Depart again".
One final element to consider is that of Friar Laurence's complicity in the relationship. He is presented throughout as a respectable, respected and compassionate character, so when he marries the couple we can assume that he is convinced by Romeo's feelings. However, Friar Laurence is desperate for the Montagues and Capulates to stop fighting, so by marrying Romeo and Juliet he may simply see a way out of the feud.
Through close reference of Romeo's language throughout the play we could make a strong argument to say that he is genuinely in love with Juliet, but when examined with various other elements of character and theme there is enough ambiguity to suggest that the answer may not be quite to straightforward.

It's hard to say. Is he really in love with Rosaline at the play's beginning? Probably not. The speed with which he abandons his feelings for her once he meets Juliet seems to indicate that he probably does not really love her. However, Romeo's willingness to change his behavior after meeting Juliet may serve as evidence that he does, in fact, love Juliet. He tries (at least initially) to avoid a violent engagement with Juliet's cousin Tybalt, after their marriage, even when his friend Mercutio says that he's behaving dishonorably. When he believes that Juliet has died, Romeo is not able to move on with a life in Mantua and decides, instead, to end his own life and remain with her in death. These actions all seem to point to a true depth of feeling for Juliet.