If I had a quarter for every time I say "now what's your problem?" to my Pfaff Quilt Expression 4.0, I wouldn't need it, because I'd be rich! Fortunately, I've learned how to fix, or at least work around most of its issues. These are just my experiences with this machine (well, except for the link at the bottom), and you may or may not have the same issues.
It does some things beautifully. Its special stitches are gorgeous. I love the fact that I can store patterns and phrases in its memory.
I have no idea how the buttonholes work, because the last time I made a buttonhole was, I think, in 1977.
BUT (and where shall I start)
The front of the machine slopes down, meaning any tools - thread snips, stiletto, etc., that you put beside your work all slide off onto the floor. The fix - either set them to the back on the flat part, or count picking them up off the floor as part of your daily workout. If you're going to set your rotary cutter there, close the cover! Don't let it fall onto your feet with the blade exposed!
There is a cut out infront of the bobbin plate, for the purpose of removing the bobbin cover. If you're working with a seam allowance that must go up instead of down, this little cut out will flip it the other way every time. The fix - check the direction of every seam allowance after it passes this little obstacle.
The thread cutter works great, although it is rather slow compared to my Husqvarna, but it jams up after a bit and won't cut at all. The fix - pop out the throat plate, slide the frame that covers the cutter to the right, flip up the cutter blade, and you'll probably see a lump of thread stuck to a piece of hook and loop tape. Get rid of the thread, put everything back as it was, and you should be good to go. This happens to me about twice a day, but for normal, non obsessed sewers, probably about once every 4 or 5 hours.
It doesn't feed properly going over seam allowances, because they are thicker. The fix - adjust the foot pressure. Just remember, this will reset when you turn the machine off, so you will have to readjust every time you turn it on.
The included quarter inch foot takes some getting used to. It tends to let the fabric wander away, especially if the foot pressure has been adjusted to accomodate the seam allowances. The fix - apparently there is a newer quarter inch foot for them that works well, but they always seem to be out of stock.
It frequently "eats" or pushes the beginning of a seam down into the throat plate, or just jams and won't move forward. The fix - use a scrap of fabric (I call them "pigs") at the beginning of seams. This is tedious, let me tell you, but I've become really excellent at remembering to chain piece.
It doesn't use all the thread on the bobbin. No, it's not that much thread, but hey, thread's expensive, and I'm on a budget! The fix - if you look at the bobbins (and only Pfaff ones will do), there is a ridge on the bobbin shaft. When you start to wind the bobbin, use your finger to push the thread down to the bottom part of the bobbin after the first couple of winds. This causes it to fill the bottom half of the bobbin first, and the "bobbin out" sensor won't go off until you're down to those first couple of winds, letting you use almost all of the bobbin thread.
I haven't tried free motion quilting with it; I use the Husqvarna for that because it does such a beautiful job.
For a great discussion of the QE4, check this out . . . . there are some great fixes and work arounds in here:
The bottom line, for me, if I had it to do over, I wouldn't buy this machine. For the price of it, there must be a better option.