SimpleXML seems ok, but one thing that's annoying is whenever assigning/storing a value you have to typecast it - this really is a tad annoying. DOM is ok but I really like the flexibility of XPath, sometimes DOM can be a tad too much writing-wise.
I really like the selector implementation in jQuery, and the flexibility. I came across http://jquery.hohli.com/ while googling but just wanted to see if anyone had recommendations before I try it out.
I usually use SimpleXML - it does support XPath with SimpleXMLElement::xpath.
Maybe a bit more information on what you're trying to do... are you reading a whole XML file into a data structure, or just trying to grab one or two values from it?
I use SimpleXmlElement for reading and DOMDocument for creating XML. You can also look at the XMLReader as in PHP 5.1. This is a fast way to "walktrough" an XML document. PHP also provides event-based XML parsing (SAX).
If you have a good combination of read/write actions you maybe should extend the SimpleXmlElement or DOMDocument classes or write a few helper functions which do all that casting work for you.