It matters whether you have broadcasters in your region.
I personally love it because there's a couple of AM stations I regularly listen to (local NPR and sports stations) that have FM HD simulcasts. Under most conditions, the HD broadcast is more reliable, significantly cleaner, and higher fidelity.
As with all things digital though, it's generally a binary situation. Either it's working or it's not ... there isn't an in between with noisy reception or anything. It just stops. Generally in that situation it just drops to the analog broadcast. That works fine if your listing to the primary multicast (HD-1).
If you're listening to a substation though (HD-2), some radios may fall back to HD-1 and then the analog station? Problem being if you're listening to a substation, the primary is a totally different feed. Might matter the radio. Some may only switch if you're on the primary?
Either way, I think it's definitely a nice to have. If you're going to be using this car and radio for a while, it's worth the extra $50 IMO.
Oh BTW - the other nice thing is metadata. It's up to the station, but there's plenty that will have a scroll with the song title and artist, etc. that shows up on your radio.
Do speakers make a difference in quality? I have a 2007 Kia Optima with standard speakers/no infinity system.
In so far as they normally do.
HD Radio is simply a digital broadcast. The actual quality is up to the broadcaster, but assume at best it's like CD versus tape. HD won't have any noise and is crystal clear. Analog clarity will be based on proximity to the antenna and is a lot more susceptible to interference (particularly AM which has lots of issues with physical obstructions).