The WRT54G was a widely popular and successful router for its reliability and hackability, and now Linksys wants to bring some of that love back in an updated, modern form with the WRT1900AC.
Not surprisingly, the WRT1900AC has an easy and friendly user interface, one that TrustedReviews calls "the most intuitive router UI." PC World says part of the reason it's so friendly is that is has "few features to configure" and the firmware is "very much a bare-bones affair," which may be an issue for some power users. However, for those who do want more will be happy to know that Linksys is planning to provide support for OpenWRT, adding back in the missing features users want.
On the performance front, you would be correct in thinking this router is fast due to its AC branding, and CNET says the WRT1900AC "proves itself right out of the box," calling it "one of the fastest Wi-Fi routers on the market." In testing PC Mag found it offered "excellent performance" and when paired up with another AC bridge, the WRT1900AC provides "unprecedented throughput."
The WRT1900AC is quite expensive, but great performance doesn't come cheap. If you've been missing the old Linksys and need the fastest WiFi available, you should seriously consider the WRT1900AC -- just remember that you will need to spend a bit more for AC adapters or bridges for any non-AC capable devices.