I've gone through 3 or 4 transmitters. I've paid up to a hundred bucks for a couple of them and they've all stunk.....except this one. The cheapest, most unlikely to succeed unit from Crappy Tire has performed the best for me. I tried the pricier ones, wasn't impressed so thought what the hell, try this cheap one based on the decent reviews. It's regular price is $34, but I got it on sale for $12. I have seen it as low as $9 though, and I see them on sale often. The only disadvantage I've found is the 3 or 4 pre-set frequencies you use which really hasn't been an issue. I've used and abused it for probably a year now and still works fine.
http://www.canadiantire.ca/AST/browse/7/Electronics/AudioEquipment/iPodAccessories/PRD~0374465P/Digipower%252BiPod%252BNavigator.jsp?locale=en