Wags hit the nail on the head. There are so many variables involved that are specific to each person. For instance: what kind of car do you drive? How supportive/tight are the seats? Do you drive for a long period of time each day?
Sports cars with large, tight bolsters can be murder while IWB'ing.
Personally, IWB works for me but ONLY with a really good holster do I "feel it disappear." A cheapo Uncle Mike's would turn me off to IWB forever, so I don't really recommend buying just to try. I'd try to find a used holster, or just take the leap and call it a learning experience.
It takes at least 2 weeks, in my experience, for a holster to break in and mold to your body. So give it a chance.
For me, a Milt Sparks Versa Max 2, a SIG P239, and a good belt is as close as I've come to IWB heaven. I love carrying that rig. It disappears under anything I'm likely to wear, and is fantastically comfortable to me. It's a little irritating in my tight bucket seats, but adding an inch+ of steel on my hip will do that. A belt-clipped cellphone is equally irritating to me when driving my car, so there's a frame of reference for ya. The SIG's barrel is just short enough not to bother me when I sit (I carry a 5" 1911 in a high-riding holster to alleviate that issue), and the VM 2 is so stable I know the pistol will stay put whatever I do.
Is it as comfortable as pocket carrying my Scandium J-frame snubby? Nope. But I don't mind, I have actually come to like IWB, and my pistols that I carry that way.