One should also consider how the new progressed sign stands in the chart, and how it's ruler is placed. If, for example, you have Mercury placed weakly, and you don't take on the nature of Mercury or Gemini much at all, the progression might not be very strong. However, if you are "Hermes incarnate" and identify strongly with Mercury and have a strongly placed Mercury and/or prominent placements in Gemini, possibly the progression will help accentuate the qualities and you might notice it alot more! What about the elements? Is air strong in your chart? If so, progressing to an air sign might be alot more notable.
As always, you can't take anything alone, consider the whole chart.
A friend of mine had her Sun do the Tau->Gem progression last year. She has definitely become more restless, and is starting to break out of her "comfort zone" (a very Taurean thing) and is considering new professions and has indicated more than once a desire to learn new things and expand her mind (very much like a Gemini!) -- She's only into the 2nd degree of Gemini now, so we'll see how it plays out. I'm guessing a Solar progression will be much more notable, especially from an outside viewpoint. However, if you experience similar things, don't be too surprised. The ascendant focuses the experiences of the chart, so if you find yourself on the Gemini bandwagon of experiences, roll with it...
I will agree the progressed angles contacting planets is really powerful, especially in tandem with other indicating factors. For me, Uranus showed this. My introduction to astrology came along with my MC progressing to Uranus, and basically my whole being took on a strongly Uranian tone with this progression. I'm also about to have my ASC progress from Cap->Aqu. With Uranus ruling Aquarius, it's hard to say what's attributed to the ascendant progression and the MC/Uranus progression, and what has to do with Uranus entering my 3rd house of "everyday thought" -- All I can say is the transits of the past few years have "Uranianized" me, lol.
It's been a fun ride, and I'm hoping for another ride on the Uranian rollercoaster as my ASC flips this year.
I guess I'll be a Neptunian in my old age... In about 30 years my ASC progresses to Pisces while my MC progresses to Neptune. Hopefully then I will gain some psychic/intuitional ability and not as nasty ailment/infection leading to my dissolution <giggle> -- you can bet I'll be watching my health around that time, especially with Neptune in my 12th!