Short answer? Absolutely worth it.
I would say it depends what you want out of IMVU, but honestly even if you're just looking for good conversation and nothing else, it's still worth it. I haven't touched GA rooms in years, which isn't to say they're bad ..but I prefer the added layer of comfort that AP users are probably grown adults. Also a lot of my outfits can end up with AP elements and I don't want to mess around making separate outfits for GA rooms so that people can see them. Add to that I find a lot of AP clothing pretty high quality, especially in days gone and I don't see any reason not to have AP.
Though I will say try and get it on sale, if you can. I don't necessarily think it's worth the full $20 price tag unless you're getting it in a bundle.