I will recommend that you be careful which lenses you use with them, there are some optical designs that simply aren't well suited to having much more than 45.46mm of space behind their rear element, and it can cause some pretty odd behaviour from otherwise perfectly satisfactory lenses. I had one such issue arise from using my Sigma 180mm f/3.5 Macro lens (a non DG version, mind you) that developed a flare issue that only reared its ugly head at 1:1 focus with 25mm extension tube. It simply revealed a issue with internal reflection within the lens - the extension tube simply made it stand out more.
- this is the flare problem I encountered with my Sigma 180mm f/3.5 APO EX [IF] these spots would appear right in the middle of the frame.
So all I can say is be mindful of the lenses you use with them. Personally I use exclusively manual lenses on extension tubes, I never use my DA or DA* lenses on them - the reason for that is there is a big difference in way the aperture mechanism works between the older K~FA series lenses and the newer D-FA~DA* lenses that do not have aperture rings. I assure you there isn't likely to be any mechanical problems - it is simply the fact that DA and DA* lenses the camera controls the aperture electronically, with manual lenses you have all the control over the aperture used. The best way around this problem is to avoid it altogether. Also using extension tubes can cause the viewfinder to become darker and darker as you add more tubes, having a fast lens certainly comes in handy here - and having a fast lens helps with nailing the focus. Speaking of focusing I have been rather lucky with my extension tubes they are able to be used with all pentax K AF mount lenses with full auto-focus, and full automatic metering however, I never make use of this faculty, because at 1:1 and beyond even manually focusing can become a real challenge - and even the most advanced AF camera I own ( Nikon D4) cannot focus consistently at 1:1 with my Nikkor 105mm f/2.8G VR ED at f/2.8.