I've been to Dominica 8 times since my first trip in 1997. I'll be there again this February. It is one of my favorite places on Earth.
Everyone's experiences and expectations are different. I don't think generalizing is helpful. I personally would not dream of spending any time in Dominica without a 4X4 rental (about $270 US per week). Driving is challenging, but the rewards are worth it. Naturally, you must be a really good driver, adaptable, able to drive on the left , and not inclined to panic simply because the vertical terrain and frequent rains wash things away and roll big rocks across the road. This is really no big deal.
The island is almost supernaturally beautiful, endlessly enchanting, relatively untouched. Compared to Dominica, Cozumel is like New York City. The diving in Dominica is usually pristine, and spectacular in some places. It's not the best I've experienced in the Caribbean, but it's among the top 4 or 5 most enjoyable. Dominica is really just the top of a volcanic structure, so there are few real beaches, and the coral reef zone in most places is a narrow band along a coast that can plunge to thousands of feet of depth only a couple of miles from shore. There is more rock than coral, but the water is beautiful, the corals are very healthy, and the sealife is abundant. Because of the proximity of abyssal depths, you may see many unusual things.
In addition to the dive operators already mentioned, I'm familiar with Cabrits Divers in Portsmouth, in the north, and Carib Dive Club, mid-island, near Salisbury. I've always had a pleasant time with Carib. The others can be hit or miss, depending on who is running things that week, who you are diving with, etc. I think this is generally true everywhere.
Dominica is quite poor, and its towns have a slightly shabby aspect. It is a very safe and stable place, though, and its people are usually extremely friendly. No one bothers you or trys to sell you anything. Most of the island is mountainous, steep, dense with thick rainforest, and uninhabited. The roads vary from fair to hair-curlingly hazardous. I love it all. The air is the purest I have ever breathed. There are huge parrots, Boas, Iguanas, vistas that seem like something out of Tolkien's Middle Earth, warm crystal clear rivers, dozens of waterfalls, hot springs, deep lush valleys, simply amazing things. Visiting Dominica and not spending at least as much time exploring topside as diving would be like going to Paris and seeing nothing but the gourmet restaurants. A wonderful experience, but you will have missed experiencing one of the world's wonderful places, and there are fewer of them every time I turn around.
Dominica is not for everyone. The food is only fair, and there is almost no tourism infrastructure. The massive high mountains draw rain clouds (that's why they have a rainforest), and it is frequenly slightly overcast. It rains at least a little almost every day, but magically, the sea is always clear, and the cloudbursts are usually brief. You'll see rainbows and double rainbows, brilliant stars at night. There are very few sandy beaches, and those that exist fringe turbulent often dangerous seas. It's not a resort type place, really. It's something else entirely. January is usually a good month, weatherwise.
