I had to use a 203k when I bought my home. I just had such a horrible experience with it that I'd just rather avoid it for now. And yes, it really is a long process.
If you're going to use contractors for your flip/rehab, then I can completely see it being quite useful. But be sure to visit the job site several times a week. If you don't, there's a good chance that you won't make your deadline.
Half, if not more, of the construction crew couldn't do the basics; installing a sink, plugging in the stove, etc. I'm not joking. $20k for the rehab and we ended up doing some of the work ourselves, so we could finally move into a house that we've been paying for.
Also, make sure that the person working on your loan REALLY knows how to work on a 203k. It's really frustrating when they state they have worked on these types of loans, but come to find out that they've only done it once. Which often left us in the dark because couldn't answer questions for us.