I haven't tried XCode i'll have a look after this post. I have Smultron, TextWrangler and TextEdit, but they don't check syntax in a fashion i can get used to yet. (Probably mostly because i'm a Notepad++ Fanboi.)
I just find error checking so fast in NP++ and often near impossible in others. The way the colouring works as well is phenomenal. If i'm honest though, as a basic coder and a basic user of NP++ i could likely get over my issue with time.
Something very simple, which i find infuriating for example. If i highlight a div close tag in a php page, it will highlight the open tag regardless of where it is within the document, which doesn't happen in any editor for the mac i have tried. It will do it as html, but if it's php it doesn't check the html within, which means for speed, i'm best off doing it on my pc.
I'll check Xcode now though, because if i find a decent enough solution my pc can likely rot.
EDIT: Looks like you have to be a registered mac developer.