I will personally not buy any more of the cheapo mechanical timers, and here is why:
Where I live we lose power frequently. Every time the power goes, it throws off the 24-hr cycle, which means after I catch on to it, I have to go through the process of adjusting them back to where they need, and my timers are not the most accessible things in the world usually.
Also, I have two timers on one of my tanks. This is so that I can have a sunrise/noon burst/sunset kind of thing happening. Well guess what? Not all timers are made equal, and even when I don't lose any power, after a little while goes by, the timers drift, and are no longer on the same 24 hr cycle!
On my 10g aquarium I have a single digital timer. It has a little battery backup thingy on it. I've never had to reset it or adjust it yet. The only time I ever have to do anything to it is when daylight savings time begins or ends. I've had it for over 2 years now, and still it is going strong, accurately keeping time, and even during hurricanes and big storms where we LOSE POWER bigtime, the battery has kept its time.
So, I've decided to can all the mechanical timers and use them for christmas lights. And now I will exclusively use digital timers. The convenience alone is worth millions to me, yet these timers cost about $11 at Target.
*For those non-observant ones of you out there: make sure you get the 3-prong timers!!! IME, most light fixtures (except for the 27w Hampton Bay desklamp) have grounded 3-prong cords.