I personally don't have an ion(yet, hopefully) but I've done a few audio installs in my day. Without the RCA aux out it's going to be tough to be able to get the sound directly through to the HU. Would it be worth it? Simple answer: no. I'd honestly rather spend my time and money on a nice aftermarket deck (but then again, I'm also planning to install a few more aftermarket audio items

) Like others have said, FM transmitters are not that bad. If you feel like spending a few bucks, that is. The monster cable one is VERY good. But guess what, at over $100cdn for it... that is what steers me more and more towards an aftermarket deck. I assume most of you prefer a stock, cleaner look, but there are definately trade-offs. Even cheap decks should have some sort of aux rca out. That way you can get a clean signal straight from your headphone jack, or if you can find the cable, I've heard there is one that goes from the dock port into RCA's... YMMV.
I personally have the griffin itrip mini... and short story is: I don't use it. A friend of mine has the monster cable adapter, but it has never appealed to me, not just because of the price, but the wires etc. bother me. It DOES come with a lighter adapter, which is definately nice, but I installed a power inverter, and prefer that any day over having car adapters for my phone, ipod, laptop, etc. etc.
The itrip, in my situation, does not have very good reception. Depending on the way I angle my ipod, I can get alot of interference. The sound is fine for talk shows and low volume listening, if you get decent reception that is. If you are in a busier area, I just never seemed satisfied with it. It was "okay," but nowhere near CD quality IMO. Also, the itrip does cause my mini to use double the power(using tom's math of guessology). I wasn't too concerned about power consumption, as, I have the 2nd gen which has excellent battery life anyway. 8 hrs is more than enough, usually. Needless to say, I've used the thing for about 2 weeks and a total of 20hrs, roughly (used it on a trip to Prince George and back) If you live in an area that isn't too busy, in a smaller city, the thing is pretty decent. The sound is bearable in low traffic area's, but a bit of a hassle to find the right frequency and setup in more congested places. If you don't want to cut into your stock system, or feel like having an aftermarket deck, this one is fine for low volume listening, and is fine for everyday use, as it just sits right on and fits really well, it's easy to take on and off, carry along with you, and use for other things than your car. The monster cable one is actually really good. I haven't had personal experience with it for extended times, but what I've heard it's probably the best FM transmitter you are going to find. Again though, the thing has wires and would probably just be a pain in the ass if you don't use it on longer trips more often. I'm one to not leave things hanging around that would entice someone to steal the car anymore than I have to, and I think that's why I've never lost a stereo... yet. Anyway, get the iTrip simply because it just pops right ontop, clean and sexy, which, IMO, is the only reason why people buy an ipod in the first place

. (kiddin')