Granted, it's currently better than OmniWeb (which doesn't have CSS support so that app is a loser in my opinion) but that doesn't mean it doesn't suck.
some of my reasons for considering IE 5.1 preview to suck:
1. when downloading files, IE grinds to a halt and can cause poor performance in across other apps (spinning rainbow)
2. when downloading files, IE doesn't always show that its making progress. On occasion I've noticed that the Download Manager will state that it's waiting for bytes, but the icon on the desktop is updating with its progress.
3. complex tables and css pages tend to flip it out. Often will not draw large sections of a web page and will have to roll the mouse over stuff to make it visible. on css (and some normal pages) rolling over links will cause the link the flicker -- not all the time, but it does happen. For an example, visit this site:
http://www.duarte.com -- I spent a LOT of time developing this site making sure it looks good on Mac/PC and IE/Nav but the Preview version just chokes on it. The html is sound, too, so don't blame me
4. poor implementation of the OS X checkboxes and the like. will often find checkboxes on the wrong color or bordered in black (eww)
5. "long load times" -- the app is probably loading quickly, but doesn't display stuff until its most of the way done with the page (there may be a preference to adjust that) which gives the impression that its loading in fits and spurts
I'm sure there's more, but I'm away from X until tomorrow