A few days ago I moved from a WRT54GL running Tomato 1.25 ND to an RT-N16.
I had cloned the entire configuration (offsite), I went to the site, replaced the old router with the new, and checked with the most remote client in the large house. I did a few checks with my phone. That was it. About 10 minutes, including answering lots of questions.
I think of the N16 as very similar to the 54GL — just bigger, faster, and more versatile (has USB), but most important (for me) it runs better software.
(It's a nuisance that despite K26 and plenty RAM, the numbers are still 32 bit, which means that the fundamental network byte counters break in a matter of hours, if not minutes. And the shell and most essential commands (ping, nc) are still crippled. But most people don't care about such things, and those who do have options.)
The GUI now does a lot that had to be done in the Shell before, and everything is more solid and stable.
So, if the question is "what is a good replacement for (or a good step up from) a WRT54GL?", the RT-N16 seems a plausible answer.
The difference is a real treat, as far as I am concerned.