You are going to get a lot of different views on this issue. I have a set of Kreitler rollers with a small diameter roller (2.25). Kreitler (USA) makes the best rollers in the world. They are very smooth and are designed to last a lifetime. Sportcrafters also makes very good rollers in a slightly larger diameter (3.25).
There is a learning curve to riding rollers. Start in the doorway and work into it slowly. Depending on which rollers you purchase, it may take a few sessions.
The diameter of the roller will vary the amount of resistance. Smaller diameter rollers offer greater resistance without the need for a headwind or other resistance device. Medium diameter rollers (3.0 - 3.5) offer some resistance. Larger diameter rollers (4.5) offer less resistance and may eventually require the addition of a resistance device.
Rollers have significantly improved my spin and balance. The smaller diameter also makes for a significant workout.
I just bought Sportcrafters because I wasn't sure if I'd really find them useful and wanted to save a $100 over the "gold standard" Kreitler. I've been quite pleased. I was up and riding pretty quick using the doorway technique. I've since graduated to a couch on one side.
More impressive has been the improvement they've made in a very short time. They expose every ripple, imbalance, wobble, etc. in your pedaling. After only two or three sessions, I noticed how much more smooth my pedaling and steering were on the road. I also have this sense of "muscle memory" while riding when I need to spin up a bit of what it feels like on the rollers.
Couple of suggestions
1. Start without clipping in. I actually used Merrel sports klogs because they had "quick release" in that I could get out of them as well as off the bike quickly. the thicker soles also helped.
2. Some say to lower the seat a little when starting...thicker soles do the same thing.
3. Don't look down...focus straight ahead.
4. Steer, don't lean. Leaning causes wild isolations. Try moving alternately to the right and left side of the roller as a way to learn control once your stable.
5. Relax your shoulders and arms and really feel the spin.
I've really been pleased and find that even getting on them as a ten minute warmup before a road ride is useful.
I just got a set of minoura rollers a couple days ago. It was a little strange at first, but I got the hang of them after a couple minutes. Looking straight ahead definitely helps a lot. When I look down at the rollers I start moving all over the place. Also, I found it much easier to ride them while clipped in.. I had a hard time keeping my spin smooth while not wearing clipless shoes.
Also, I think if I had done it again, I'd have gone with 3" kreitlers. The minouras don't offer too much resistance without the mag unit (mine came without it), and it's actually pretty bumpy and loud. I hope all that vibration and whirring doesn't tranfer through the floor to the neighbors below me..