Gone:
The easiest to explain this is that celexa is a selective seratonin reuptake inhibitor (same as prozac, lexapro, paxil etc). Minus the scientific mumbojumbo, taking the drug basically results in a much higher level of seratonin in your brain; one of the many chemicals that control your mood/level of general happiness. The half life of Celexa in the human body is like 30 to 70 hours, so as you take it on a 24 hour/daily basis, the levels of seratonin in your brain are slowly raised, making you feel happier and less depressed.
Many recreational drugs have similar but more extreme effects. Methylenedioxy-methylamphetime (ecstasy) also causes seratonin levels in the brain to surge, resulting in feelings of happiness, pleasure and bliss.
Yes you can walk in a straight line lol. You can function perfectly normally, you will simply feel happy for no explicit reason.
As others have said, everyone reacts differently to different medications. After a week of taking it you could feel high as kite, happy and care free, or you could feel nothing at all.
I used to take Zoloft and it did absolutely nothing for me. But Lexapro and Celexa have both helped me feel awesome all day long for a while now.