I haven't taken the CELTA but have done a lot of research on it. I would say that it depends on the availability of your time/money, and your confidence in your abilities to teach for the ETA. Personally, I have a lot of tutoring experience but no classroom teaching experience, and I'm a bit of a shy bird, so I think the CELTA would really benefit me if I could take it before the ETA. From what I've heard, it really improves your confidence and gives you a lot of tools to work with so you won't be floundering when you have to give your first lessons.
With that said, it does cost money -- usually around $2500. It's cheaper if you take it outside the US but in that case you might have to pay for a flight so I don't know if that helps much. So basically, if you have the time/money and don't feel 100% confident about classroom teaching already, I would definitely do it. It can't hurt you and will probably help reduce some initial stress with the ETA. It's also a respected certification and even if you don't go into English teaching as a career, could be applicable to any career involving public speaking, teaching, cross-cultural/linguistic communication, etc. There are also cheaper TOEFL courses than the CELTA but the CELTA is the best and most worth doing.
Hope this helps!