I agree that everyone is different but I feel I must warn you I was on lexapro for 6 years and for me it worked for a while but it did cause dizziness the problem for me was getting off of it. withdraw was wicked and I felt like I had a severe flu for almost a month. I would talk to my dr. about drugs with less withdraw symptoms. However if your dr. still suggests lexapro then maybe it will work good for you.
I'm not a psychiatrist, but I know mine started me on 5mg Lexapro and worked up from there. Most people only take 10mg, some need 20, but starting at 20 seems high to me. On the other hand, he or she might be trying to cushion any withdrawal from the Effexor. Talk to your psychiatrist, feel her out, learn if he knows anything about the drugs your working with. Most of them don't. You need a good one. When you go on one drug while weaning from another, it's difficult to tell side effects from the new one from withdrawal from the old one. As for side effects, cj is right, we're all different. None of us knows if you'll have the same ones we did. Lexapro has the reputation of having far fewer problems than Effexor, though, if that helps. As for the advil, there is a caution against using it with ssris and snris because of potential for bleeding problems, but occasional use probably won't be a problem. Discuss this with your psychiatrist; if you don't get a really informed answer, get a different psychiatrist.
Medications will affect people in different ways, but I am a big proponent of medications like Lexapro as long as it is combined with talk therapy...in my opinion, only by gaining the long term knowledge about why you are anxious can you get long term relief from this. I am glad that you are seeing a psychiatrist; they usually have much more knowledge with these kinds of medications. When I started on a medication similar to Lexapro, I did have some side effects in the beginning, but they went away after about a month or so...keep us posted!