Mounting Ext3 filesystem with OS X 10.7.4
I have successfully used Paragon's ExtFS driver in OS X Mavericks (and Mountain Lion previously) with full read/write access. This has worked for me with ext3 and ext4 file systems.

My only concern would be if you are mounting your Linux root (or any other system root) and you are therefore able to touch sensitive files (/etc/ comes to mind) that may stop working (Linux does not boot, etc) if the wrong permissions get applied. The solution to this is to mount read-only but I cannot say that Paragon's software really has that as a built-in option. The default mode (as is with OS X in general, except for the built-in NTFS driver) is read/write.

Also, do not expect performance in any case. I tried to use my external drive with iTunes and the performance was intolerable.