I had issues getting used to the 6000s at first, but with a LOT of trial and error managed to find a near perfect fit.
My first device was the 3000, and as is common I relied fairly heavily (twice a day) on lube - silicon, not water based. My record for weaing was just under a month, with bi-weekly release for cleanings. Security was an issue however. I decided to try the 6000s, as I was short in the tube of the 3000 and thought a smaller device would be both more comfortable and more secure for long term wear.
When I ordered the 6000s, it came with a note stating the no lube was the best method. I was doubtful to say the least, and sure enough my first few tries only lasted a few days before the ring chaffed me raw. I must have tried dozens of combinations before I found one that worked, and sure enough without lube. (hint: use the cut off end of a pair of nylons to pull yourself through to get in the tube lube free)
Currently I am only on day 11 of my non-lube 6000s lockup, but there have been no issues with comfort for long term wear at all, and according to my Mistress/Wife I will be in until at least April. Hygiene is also much easier without lube buildup to contend with. I highly recommend the cleaning kit from KFH, if only for the sponge tipped swabs. They have been so useful that there is no longer a need to be released for cleaning - which means my 11 days, although not a record in itself, IS a record (for me) for continuous wear.
I must add that I am using the KSD-G3. This was perhaps the hardest part of the whole process for me. With the smallest size, I was able to pull back behind the KSD when flaccid, which not only made it nearly impossible to urinate, but defeated the purpose of the whole exercise (anti-pullout security). I tried the medium and large, and although they worked great for security, I would bruise quite badly from the front part digging in when having a partial erection. A few days would be fine, but any significant teasing would result in needing removal from the pain. I did try to work through it, but once bruised, even being flaccid resulted in intense pain.
The solution ended up being to heat up and reshape the smallest KSD so that the tip bent down. It appears to have enough give when excited that the bruising is minimal, and when flaccid it is down enough to keep the penis from being pulled up, but not so far down as to aggravate the bruise (much).
The result is intense pain when erect, and only mild discomfort after the fact - exactly what Mistress was aiming for. In fact, she will tease me and pinch my nipples regularly just to see me squirm from the pain.
In summary, yes it is possible to use the 6000s for long term wear (so far), but it does take quite a bit of experimentation to find the perfect fit. Also, although I know I have already said it, I cannot stress enough how much better the whole process is without the need for lube.
Best of Luck!
-HiH