I'm using them, have been for 2 months or so, don't forget I'm a fairly new reefer so I can't say much about long term or vs other forms of carbon dosing but I'll share what I've got.
I'll go with negative first.
1. Require more flow than most other media in reactors so you'll need a larger pump. (not a big deal but don't expect the same pump you used for GFO to work for pellets)
2. Related to 1, you'll probably need to mod your reactor to get them working, or better yet a brand new reactor. The pellets tend to clump and settle if the flow isn't reasonably even. I have a nextreef MR1 and I modded it a bit to get it working, wasn't really satisfied with the result ended up getting a mod kit from the company that replaced the 1/2" tubing and connectors with 5/8" which worked a lot better with the same maxijet 1200.
3. You have to start slow, I started with 1/4 the recommended dose, took it up to 1/2 now and will increase it to 3/4 or full recommended dosage soon.
Positives:
I've been changing a bunch of stuff in my tank but when I took the GFO offline and put on the biopellets I saw a real improvement in my nitrates. My algae issues are slowly fading. It's also a lot easier to make sure it's working properly just make sure they're not clumping and add more every 4-6 months when the pellets are used up.