Good and fast, but can stall if asked to do too much.
A pretty good, fast web browser, esp. under certain circumstances, on certain versions of MacOS and on certain machines. On Snow Leopard, Safari's cache (where it stores images and other files, to quickly be able to reload pages it has already visited) would balloon into many, many GB — whether you were using the software or had quit out of it — until you manually emptied the cache, or were forced (by Safari's slowing the whole machine down to a crawl) to reboot the machine. Also, if you had too many pages open, the software would get extremely finicky, exhibiting the spinning beach ball of death for minutes at a time, and sometimes outright crashing the system.
On Lion the whole experience of Safari seemed faster, but the cache issue remained. I used it as my main browser for 4 months on a Mtn Lion machine, and although I didn't see issues with page counts (possibly because I had 16GB of memory instead of 8), I noticed that certain pages, PDF file pages in particular, were prone to making Safari "forget" all of the pages it had loaded, and asking the user if s/he wished to reload all pages. Pesky.
I have used it for about a week now with Mavericks (Mac OSX 10.9) and so far have not encountered any issues — but I also have not taxed the software too much. I remain hopeful.
Startup speed (fast)
Page loading speed.
Stability, esp. with light use.
Unlike Firefox, doesn't perform lengthy system check at startup for extension compatibility.
Better tab stacking solution for multiple pages than Chrome or Firefox.
Intuitive to use, esp. vs. Chrome (version 30-33 at least)
Ability to either not crash the software if too many pages are loaded, or warn the user before too many pages are loaded.
More specificity of where to place bookmarks, esp. when dragging the favicon to a bookmarks bar folder.
An even more improved tab stacking solution when you have a great deal of pages open More