All these previous posts have summed it all up nicely.
Let me just add my overall opinion of Merlin and why I truly believe that it is the best choice for your ASUS router. I should mention that I ran with Tomato for the Linksys WRT-54G router for nearly 5 years, and another 18 months with DD-WRT after that. When I replaced these old workhorses with a Netgear WNDR-4500, the Netgear firmware offered enough options that I didn't feel compelled to venture into these two alternate offerings for that router.
When I picked up my first 68U and flashed the latest ASUS firmware, which as bad luck would have it was the now infamous 3.0.0.4.380.1031, my brand new router's 2.4 GHz wi-fi radio stopped passing internet to any device connected to it. Unable to find a solution on the ASUS side and not suspecting my problems to be caused by this latest stock offering I returned this 68U to Amazon as a defective unit and requested another 68U. When the second brand new unit arrived, and after finding that it too was crippled by the installation of the latest ASUS firmware, I turned to Merlin's latest public release (378.56_2) as a Hail Mary attempt to fix my second broken 68U in a row! I really wanted to stay with the ASUS brand and specifically, the well regarded 68U if I could find a stable firmware offering for it.
Merlin 378.56_2 not only immediately resolved all the strange behavior coming from my 68U, I soon found out why this firmware variant is so popular with so many ASUS router owners. A popularity that spans nearly the entire product line, which considering the number of router models ASUS makes, is impressive in and of itself!
So here is my take on the OP's original question...
Merlin is unique among third party offerings in that it simply builds on the factory firmware which unlike most router brands out there, the stock ASUS firmware was created using Tomato as a foundation/starting point and grew up from there. But in my opinion has a much nicer looking, well thought out user interface than anything I have seen from Tomato, or DD-WRT for that matter. And rather than reinventing the wheel, our fearless leader, and firmware guru, RMerlin chose instead to use the existing ASUS code and design and simply add a nice collection of additional hardware features not currently being used/enabled by ASUS's stock offering.
That is Merlin in a nutshell.
When you move over to something truly third party like DD-WRT and Tomato, you loose that nice interface in exchange for getting access to pretty much every single hardware option your ASUS router model provides. A good chunk of these options and settings are stuff your average home network user doesn't really need, but because they are now given access to these things on the firmware's interface, many assume that they need to turn it on and try to figure out what it can do. That right there is where a lot of the problems start for many who attempt running with these more advanced firmwares. Some going so far into these advanced areas that they end up bricking a perfectly good piece of hardware and are then in a world of hurt for no good reason.
Merlin sits in the middle of these two worlds, and provides some additional hardware features most users can actually use and will improve their router's versatility. But not at the expense of the nice user friendly interface and most of the time, stability.
Most important of all, there is very little risk we will ever brick our ASUS router as long as Merlin is the firmware running on it.
