Choice is really yours....
Some people go all out and get a custom UI set, I prefer to pick and choose individual pieces out (most of them by Starrie Blacksky (sp?) Even tho those UI pieces are way out of date, most of them still work with HoT exp. I like the simple, functional look of her UI's. I'm not really into those all out sets that look like Medievel or Alien UI's....
Another plus to Starrie's...each expansion that comes out...if i have a UI prob in-game, i load each individual piece, to verify which one is NOT compatible and then look for a replacement that does work. With a UI set...if one doesn't work, and no update is made for newest exp....then you have to toss whole UI and find another one that does work.
That's the Pro of having individual pieces vs. a complete UI set.