I have experience with replenishing two developers: Kodak Xtol and Edwal Developer 12 (commercially available from Photographers' Formulary as simply 'Developer 12').

Both are incredibly nice developers and I have about six years of combined experience with them. The Xtol batch I used was active and stable for the five years I used it. The only thing you have to worry about is that if you don't develop any film for a couple of weeks, replenish the working solutions with about 70-100ml anyway.

After my divorce I went through a period of creativity problems, and didn't shoot/develop any film for about five months. I also didn't replenish the Xtol. It just sat in its container in the basement. Then one fine day I decided to end the spell and run some film. I mixed some fresh Xtol, replenished my 2 liter working solution with 300ml fresh Xtol, and I ran a roll through it, and it came out EXACTLY as the last time I had used it. Is that incredible or what?

Today I shoot way less film than I used to, so I have moved to developing film using a 2-bath method, but some day when I have more time for photography I'll probably do replenished developers again. Super sharp, fine grain negatives that I get better tonality in my prints from than stock or dilute developer.

Just try it, and have some fun. It's an amazingly economical way to process film too!