What Mike and Dan's insights offer is valuable but risks becoming bit cultish when opinons are presented without the insight or direct experience to back them up. Something that really grates on me is saying go out and feel it and then commenting on something or someone you've never personally felt. Don Angier, Kuroda, Ryabko and Ushiro developed their internal skills in a different manner than those being promoted here. For people posting here to conclude that different methods for achieveing internal skills are either inferior or less efficient without DIRECT experience to make such a conclusion is so obviously flawed that it borders on arrogance.
Let me try to re-state some of my perspectives. I've seen Kuroda do some things on film.... they're standard fare (some of the demo's) for ki demonstrations and he appears to do them well, so I would stipulate that he uses "internal strength". I've seen Don Angier on video and my personal opinion is that he has *some partial elements* of internal strength tied into his waza, but it's not the same thing. Ryabko I haven't felt, and since I've never felt Toby or gotten a clear indication of whether he truly knows internal strength skills, I simply and neutrally put Ryabko on "hold" until I can make an intelligent observation. Ushiro I've watched personally. He certainly has skills, but like so many teachers in so many Asian martial arts do, I think that his skills are bounded/defined by the karate that he practices, in the main, and I'm not satisfied that his particular method and approach is a comfortable fit for the way that Ueshiba or Tohei practiced. That last is just a thought and is not something I dwell on daily.
What I'm saying is that just because someone has a "name" and gives workshops, I'm sort of stuck about who does what.... and I'm not going to fly all over the world just to find out whether it's a waste of my time or not. I'm going to go by the words of people who I know have some ability to recognize real skills and/or I'm going to go by my impression of what they post (what it does to assure me that they do or do not really have enough of these skills to make it worthwhile for me to go visit). But that's just me personally and my method that I've developed for cutting through the weeds over many years.
I'd suggest once again, as other people have noted, that very simple and basic dialogue and anecdotes on AikiWeb clued me that Ushiro, Rob John, Dan, Dave Shaner, etc., had these skills. Turns out this is true, so some posting about the basics in public forums has some benefits for the general populace. Other people, I'm generally left with having to just shrug my shoulders and say "I don't know, I don't know the skills of the people who are recommending them very well, so I have to stay in neutral and not say anything".
And BTW, in some cases I *do* know the skills of some of the people recommending so-and-so as "having it", so in some cases I know that people are being recommended by someone whose opinions are, IMO, not of great value for a recommendation.
The long and short of it is that there is no clear-cut answer in this topic.... it's another morass, so be careful out there.