I have an SP89, so I am a little biased. Before I got it, I did some research on various sites (hkpro.com, etc.) to see what others thought. Most of the responses seemed to fall into two different categories. People that liked them and people that thought they were a waste of money.
My advice, buy it if you want it and have the money. They are a lot of fun to shoot and are very reliable. That, and they look really cool.
I don't have a problem with buying "impractical" guns. If I only were buying for practicality I would probably only have two guns, a bolt action rifle for deer hunting and some type of pistol for home defense. The problem is that military style firearms are fun to shoot and I have ended up with the SP89 and several AR's.
Is the price high? Sure, but there are a limited amount in this country and no more are being imported. Personally, I don't have mind spending money on guns. I have friends that spend money on things that I would consider a waste (classic cars, HDTV, etc), but it is their money.
I would rather have the MP5K-PDW, but that is not an option in MI, so I went with second best. There is always the possibility that MI may become a class III state. If that happens, I will pick up a registered auto sear and get all the good stuff ; )