Hallelujah!
No, do not expect a miracle. What happens is more subtle, but quite pleasing.

To fully understand it, you have to try it yourself of course. It will save me from using a thousand words here. All I can (dare!) say is, replacing the cork of an oboe tube with hard plastic will add an evenness and sense of smoothness and presence to any oboist's playing, amateur or professional, regardless of reed style or scrape.

What causes this improvement is the combination of less dampening of sound as well as an improved connection between oboe and reed.

See what other oboists have to say about it - read their 'reviews'.

Stability
Perhaps most important is the stability you feel playing the tubes and the evenness you gain. Stability in two different ways: Resistance and Tone stability:

Resistance: Playing a wonderful oboe solo requires the ability to maintain a constant airflow no matter what dynamic or where on the instrument this solo is. The 'ugly' or difficult notes such as fx. c# and g# should not stick out. When a great oboist is playing, you do not think: Ah, that was definitely a g# !
On a cork tube it is easier to play some tones than others, but on the Nissen Tubes, the load on your breath is generally the same throughout the register. This provides for a smooth and elegant phrasing, as well as improved intonation.

Tone: The tone quality should stay constant. Everyone can produce a nice middle register, while the lower notes are often a bit dull and the high notes thin and weak. Well, this is the way the oboe is constructed, and we spend a lot of time trying to compensate for that.
Using plastic instead of cork, the tonal quality is equalized, so low, midrange and top notes have the same 'sound'. The midrange tone quality is now available from bottom to top, so to speak. This in itself is not fantastic, but together with the equal response, or balanced resistance from the oboe, you can play up and down the register without fearing that any notes will stick out unpleasantly.

Just by exchanging cork with plastic, something wonderful definitely happens to the instrument, almost like the feeling you have when your oboe has just been returned from a major service check, when everything is again tight and delicate - you know how it is.
With cork the reeds feels like an add-on to the oboe; you have two separate units which have to work together. With plastic the reed is more integrated with the oboe; more of a unity - one instrument instead of two.

Can it be heard? you may ask. For sure. I recently did two recordings of a reed I knew to be stable, so it would not change anymore. First I recorded the reed on a standard cork tube, then two days later on a Nissen tube (the plastic was sliced in two to make it possible to glue onto a finished reed), same placing of microphone and all. There was significantly improved stability of pitch and a better focused and more resonant tone with the Nissen tube. I clearly felt this as I did the recording, but was curious whether it could actually be heard out there. It could.
Customers have from time to time commented on my sound examples where I play the tubes, guessing that I play fx. Lorée Royal or the newer Marigaux oboes, known for providing a dark, even tone throughout the register. I do not play these oboes, what they hear is the effect of the Nissen tubes.

"OK, but it sounds to me like this is only for oboists, who have a real bad technique, reeds or instrument?"
I would say it is quite opposite. The more control you have over the oboe and all aspects of playing it, the more you will enjoy this enhancement. Not to say it is not for everybody - it is, but a professional player is often more sensitive to the feedback from his instrument.
It's like driving a Ferrari. We may all be able to get it rolling, but the experienced driver will have more fun behind the wheel, and be able to get the most from the car.

"Still - if this is so fantastic, why haven't I heard about it before?"
The evolution in music instruments is slow. If it works, don't fix it - stick to tradition. Musicians are generally very conservative people when it comes to trying something new for their instrument. Some claim they aren't, but there is often a reluctance towards experimenting with new ideas, if proof of success can't be given just like that or if no collegues in the neighbourhood are using it. Some have to try it first. This idea, along with Bassoonboost, is mainly presented through the Internet, so it will take time before it is spread out.

Mechanically speaking there is also something to be gained: there is no need for cork grease, vaseline or cigarette paper for tubes, where the cork is either too large or too small or simply gone, deteriorated! The rubber rings assure a constant fit.

I have played these new tubes since 1999. Now and then I have tried cork tubes, just to remember how it was. Each time feeling like on slippery ground, the tone quality and pitch tend to drift around more easily; I am simply not in the kind of control I am now used to.

I recommend you try the tubes yourself. Remember: It does not mean you will have to change tubes or the scrape of your reeds. Go on as you are used to. The only thing different is that the cork will be gone, and some plastic has taken its place.

Is this expensive?