It used to be much larger than it is now. Generally, you'll only want it on Soraka, Fiddle, Karthus or a tank. You know, guys who need to be in the middle of everything to deal magic damage. --- Evelynn is my waifu. Help... Me...
Honestly, I don't think it's worth it on non-tanks anymore, unless you really need the MR and all of your abilities don't outrange it. --- /|\ http://i48.tinypic.com/2pzcsv7.jpg \|/ http://www.youtube.com/watch?v=vE7x0WLz7SY
It's 700 range, while Kayle's range is 525 on her AA's with R. Fury and 650 on Reckoning, meaning she'll always be in range of it.
However, it really depends on how much you're building magic damage on her. Her E does 60 bonus damage and has a .4 ap ratio, but it also has a 1.0 ad ratio. I don't know if I can recommend Abyssal over, say, Bloodthirster if you just want damage.
Unless you need the mres, in which case go for it :D --- http://i.imgur.com/7Wah6.jpg I'm actually incredibly overly passionate and...whats the word...viscous? - EltoniaX