The advantages of 3rd party firmware are many, but the important things to most people are:
1. The replacement firmware (particularly DD-WRT, OpenWRT, and those based on them) are Open Source.
2. Most of the routers out there are capable of doing more than what the commercial firmware they come with gives you access to.
3. Any number of these "extra" capabilities are of use to many people, even though naturally there are many more "extras" that the average person might never need. But then, look at what your computer can do "in total" vs. what you might ever use it for yourself.
4. Generally speaking, because of the inherent peer-review and publicly-accessible nature of Open Source software, you tend to get a more stable, secure and robust (that is, feature-filled) product as a result.
I have a WRT54G v5, which is in many ways a lesser version of the WRT54GL router suggested above. It's lesser in RAM, on-board flashable ROM, etc. However, in putting DD-WRT on it, I still benefit because of the better stability and greater feature set.
Personally, I wouldn't recommend any Linksys product, except perhaps the GL version of their routers, mostly because of the reduced RAM/ROM capacity (the reduced RAM contributes to more frequent out-of-memory related crashes) but also because generally their products are not as stable overall as they used to be, particularly before Cisco acquired Linksys.
I've heard a lot of very good things about Buffalo's WHR-HP-G54 units, and there are others, but what you'd do well to do instead of just listening to us here at MacRumors.com prattle on about routers is to go to DD-WRT's
Supported Devices page, because amongst other things it will tell you two very important things:
- Which routers are ranked better than others by users such as yourself;
- What the specs of each known router are, so you can make a better, more educated purchasing decision even if you choose not to flash your new router over to DD-WRT.
I am a technologically savvy person; nevertheless like anyone else I seek to do things the smart way, not the hard way, and I have found that by putting the DD-WRT firmware on my router, I am able to administrate my home network a LOT easier than I might be able to do otherwise.