My dad was on celexa for about 5 years after being on zoloft and paxil. He didn't have any problems switching, however he tenRAB not to show withdrawal symptoms from anything (he stopped smoking cold turkey no problem back in 1988, he stopped paxil cold turkey no problem, etc...). Anyway, it worked for him up until the past year or so and it just stopped working.
He's now taking something else (it's a new AD, but I can't remeraber the name) and doing well.
My mom's also on celexa and it's working great for her, but she hasn't been on it for a long time (only 2 years or so), so I'm not sure if it'll stop working for her or not.