For those longtime Mac users, first iterations of Mac OS X (10.x.0) has historically been rough around the edges, filled with bugs and glitches. For example, 10.3.0 (Panther) was famous for causing FireWire hard drives to lose all data and be wiped, 10.5.0 (Leopard) was just plain buggy, and 10.6.0 (Snow Leopard) was known to have guest user accounts wipe the main user account on the computer. For me, I began with 10.7.2 (Lion) with my MBP 2011, and to me, 10.8.0 (Mountain Lion) was very, very smooth and refined. I'd have to say 10.9.0 (Mavericks) is probably the worst, IME. It's rough around the edges and have had some bad UI glitches. Curious for what you think!