I had a batch of D-23 on the go for 2-1/2 years, replenishing with DK-25R; Kodak recommends replenishing for no more that 25 rolls/quart before dumping the developer. The developer was in a (nominal) 1-gallon glass jug, which actually held 4.06 litres when filled to the rim. I had processed about the equivalent of 75 rolls (of the approximately 107 which would have represented the maximum capacity) when I dropped the jug and it shattered! Before I made this mix I had previously been using a mix of D-23 for nearly four years, and had processed nearly the maximum amount of film. I only dumped the developer because when I pushed four rolls of film, with the developer heated to 100F, little bits of the emulsion came off in it!
The trick for replenishing, for me, seems to be to keep the developer jug filled to the brim and to not mix up too much replenisher at a time. I usually mix 500mL of DK-25R, which is good for about 22 rolls of 120 or 35mm 36-exposure film. D-23 has 100g/L of sodium sulfite, which may also explain why it lasts as long as it does. I pour the developer just used for processing into a holding container, add the replenisher to the developer jug, and top with the used developer; any excess I discard. I then cap and invert the jug several times to mix the replenisher in. With D-23 I get a sludge at the bottom of the jug which moves around when I invert the jug and settles back to the bottom in time. I have developed film with freshly replenished developer and I have never had problems (that I could see) with the sludge getting on the film. I expect that it is rinsed off, if it sticks to the film, in the stop bath and fix, and comes off totally in the wash. It can be filtered out, but I usually don't bother.
I use the same procedure with Ansco 17M. I use a derivative, which I has 2.5x as much sodium metaborate in the mix; the instructions allow for up to 10x the amount. It is in a plastic jug!
I would say that replenished systems definitely impart somewhat "different" characteristics and that they are well worth exploring.
I very much enjoyed learning and working with my replenished Xtol and got negatives that were really nice and it was really economical.
I now very much enjoy my DD-X and the convenience it provides and I get negatives that are every bit as nice IMO.
Sure, DD-X costs me a bit extra compared to replenished Xtol, but a $10 or $20 difference in real cost over 6 months is nearly insignificant when amortized over that same time.