I have used several FM transmitters over the years. they used to work much better, but the FCC had the power of them dialed back so they are not as strong as they once were. You will get bleed through from local stations that broadcast on the same frequency, so you may need to change the setting on your transmitter as you travel around. As you would guess the sound quality is not as good as you would get with a wired solution.
The best type to get (IMHO) is the type that is inline, which is to say they plug into your Head unit antenna slot and then your car antenna plugs into it, this give you very good sound quality (matches what a radio station sounds like), but you need to remove your radio, and if you do that you may be able to make a wired aux input work then.
Really it depends on the person and how picky you are about your sound quality, it may be good enough for you or you may not like it at all. I have an XM radio in my van that uses an FM transmitter (not the in-line) type and I am happy with the sound, I also use an adapter that replace the factory CD changer with an Aux input (from iSimple I think) that I run my iPhone through and this sounds better than the XM radio input.