For me, the decision to move to mechanical keyboards was primarily economic.
I was going through Logitech Wave keyboards every 5 - 6 months. The problem I have always had with membrane boards is that when they get some use, the keys start sticking on the posts. So, if you don't strike the key exactly in the center, the key would sometimes won't register. To compensate for this, you subconsciously start hammering the keys to make up for the keys that stick.
My first mech board was a Das Ultimate with MX Browns. I used that as my only keyboard for over three years. When I finally gave it to a buddy, the key switches still actuated like they did when they were new. (The only reason I gave it away is because I moved to Topre keyboards.)
So, if you add up three years of $50 membrane keyboards replaced every six months, I would have spent $300 over those three years. That $130 Das now looks pretty inexpensive.
Aside from the economic advantages to mechanical keyboards, there're other factors to consider...
Comfort - they're arguably easier on the fingers and joints since you're not hammering keys.
Options - you can select a switch weight as well as tactility (or lack thereof if you prefer a linear key switch) that suits your typing style and preference.
Productivity - I have fewer typing mistakes on a mechanical board. That's partly due to the fact that the keystroke registers approximately half way into actuation.
Enjoyment factor - I can honestly say I enjoy typing on a mechanical board far more than a membrane board.
The above reasons are subjective. Only you can decide if those reasons justify the additional cost of a mechanical keyboard.
However, the economic argument is quite objective for a heavy computer user. In fact, if you're a heavy computer user, it would be financially foolish to NOT to move to a mechanical keyboard.
