I found them unwatchable. I've made it about 20-30 minutes a couple times and gave up. Bad acting, bad cgi, terrible costumes, terrible sets etc.
I enjoyed them, perhaps mostly for the story itself. I just love the Dune universe.
But you also have to remember the circumstances: they were produced by and aired on Sci-Fi Channel (now SeeFee, err.. SyFy) circa 2000 (2003 for Children of Dune). I don't recall the acting being terrible but it's been a long time.
This was before the revolution in high-caliber cable channel productions, when production and budget were minimal. Each of the mini-series apparently were shot on a $20M budget (each, not total). For what would otherwise be a series of epics, that's probably too paltry to expect a Hollywood-level final product. We're spoiled now.
And hell, both Dune and Children of Dune won Emmy Awards for Outstanding Special Visual Effects in a miniseries or movie (Dune also won for Outstanding Cinematography). They looked incredibly dated the last time I did watch any of it, but
Also: woah, I remember them being around 4 hours. They were aired as a six-hour series, and actual runtime puts them close to a four and a half hour runtime (265 and 266 minutes). Directors Cut for Dune was 5 minutes shy of a five hour runtime at 295 minutes. I felt they did a great job, especially looking back at the production and budget. Hollywood quality? Hell no... cheap B movie quality perhaps.