RDS certainly is cheaper to implement in radios than HD and to a degree is more robust, in that it doesn't really degrade the analog signal, coverage or affect sound quality.
As Chuck notes RDS is standard in most European cars and as such, many of the higher end German cars sold here have had RDS for years. GM is also a big supporter of it; even the stripper Pontiac G6 I rented in LA had RDS with a dedicated button with a musical note on it to flip the display to the R-Text field. VW is finally on board with their newer radios, too. The holdouts are still the Asians… AFAIK they don't use RDS at all in Korea or Japan, but I know of at least one Lexus model with RDS.
Stations that implement it usually do a basic set of features. It's our radios that seem to lack the ability to use them. For example, most stations offer the R-Text field (even if it duplicates the station name field, which is supposed to be static but rarely is), the clock set feature and the format feature.
My experience is that the clock time sent is never accurate and when a station flips formats, the PTY is almost never changed or is just not accurate at all.
Lesser implemented features include the auto frequency switch, the traffic announcement override, enhanced other networks and the traffic message channel. TMC is used by many Clear Channel stations to pass traffic data to GPS navigation devices but I don't think anyone else really uses it. I don't even know what EON really does. I've never heard of anyone actually using the traffic message/announcement switch even though it's supported by some RDS encoders.
Since we don't usually have networks in the US, the AF auto frequency feature is not widely used. One that does use it would be on Mississippi's public radio network, which reaches the entire state. You can travel between stations and radios will switch from one station to the other, automatically. I tried it on the only radio I have that supports that (a Sony Ericsson Walkman cell phone of all things) and it worked pretty well. It would even overwrite the station's preset with whatever frequency was strongest. No matter where I went, preset #1 was always MPB!
Somewhat ironically, the new technology (HD) actually doesn't support most of these features and if you listen to MPB in HD, your radio won't switch automatically even if the RDS supports it. D'oh.
There was also a country station in Mississippi that was running a translator and both had RDS and used AF to switch between the two.