My local lab has a Imacon scanner and shows some great results in scanning negatives for extremem blowups (will do up to 4x5 for a reasonable price). I like that it is touchless, but I wonder how it compares to a drum scan.
Like most things, it depends. It depends on your film, your exposure, your image, what you think constitues "extreme" and countless other things. IOW, the only way to really tell what will work for you is to get scans made from the same film both ways, make prints from both scans, and compare the prints. NOTE: comparing scans on a monitor is highly problematic.
For context, I'm a drum scanner operator, so I'm not an unbiased source

What I have found is that drum scans give you two things you can't get from other scanners. One is the technical improvements of PMTs over CCDs - the very sharp imaging and huge density range that you can't get any other way (although the professional flatbeds get better every year). The other is the more subjective improvements you get from fluid mounting on a curved drum. I think each of these is worth about the same amount in the final print.
Of course, the smaller the enlargement the less these things matter. It's only after around 8-10x enlargement that drum scans become fairly clearly better than scans from other devices. Even then it depends on the image, the film, the exposure and processing, etc. Even a drum scanner can't make a silk purse out of a sow's ear.
In the end however, how a scan from an Imacon compares to a drum scan is something only you can decide. Why guess when you can so easily know? Do the tests and find out which you like better.