I notice a little more walking speed from munching hairball flowers, and silvertongue definitely works. But I'm not sure about yellow crumb, don't understand gandlevery at all, and I think rubeweed is a cruel joke.
To notice yellow crumb, you need to be
a) watching your skill speed before and as you munch it, and
b) not be learning at full speed already.
It takes a minute or two off of your skill time. Not worth the wait time, imo.
Gandlevery gives you "an awakening energy boost", which should be very clearly visible when you munch it.
I also didn't notice a difference with rubeweed, but others swear by it. It must technically increase the odds, but without specific numbers, it is hard to know whether or not it is worthwhile.
My favorite is definitely silvertongue. The results are completely, 100% predictable. It always gives a result (I've had the effect last overnight if I logged off), always the same result of (+5%? I forget the number) added to your gains.
That is all I would grow, were I an herb-house citizen.
I like them because they are pretty. I munch on a silvertongue just before completing a completest, but I haven't paid too close attention to the numbers.
Gandlevery is a slow energy boost over an extended period of time. I'm not convinced the odds boost of rubeweed is worth growing it. Before I moved out of my dark house (I loved it but I wanted to grow something other than herbs and Egg Plant), I would typically plant my six plots with two gandlevery, two yellow crumb, and two silvertongue.
Silvertongue is a good consistent benefit, and yes, hairball will increase your walking speed (great for when in the nostalgia lands). Purple is cute about once, and does provide some xp, but pretty useless otherwise. Grandlevery does an energy boost, but not worth much, and yellow crumb does a minor time improvement to skills but about as useful as pungent sunrises (which isn't much either). Rubeweed is extremely inconsistent...almost like Rube's trades now that they nerfed it. Sometimes it seems to help with drops other times, I think it actually nerfs the drops...
"Mining gem drops: Once you get your first gem from mining in a game day, your chances of getting another gem will be reduced slightly. With each gem thereafter, the rate will continue to decrease (but will never reach 0)."
When it comes to mining drops, I wonder if rubeweed would be more effective at the beginning of one's day. I have been using rubeweed while mining and have had some good times, so I will test this further ... but non-scientifcally and without tables, because I am a non-mathy glitch. :/
I tried Rubeweed at the start of a visit to the spice trees in Ix. Out of all my harvests I got 3 drops of extra items. Two were just food, but the first was a music block and I got two of the same in quick succession. Good enough for me to give it another try, at least.
I try to make sure that I always have the silvertongue effect active, because I think it works for both quests and achievements? So since I don't know when some achievements will pop up I like to have it active just in case.
I try to have rubeweed around and munch it when I'm on a harvesting spree and remember to, but as others have said it seems kinda inconsistent. Sometimes I appear to get drops from several trees in a row (though this could also be due to my skill level), other times I get nothing.
I didn't notice much, if any difference to my walking speed when I tried hairball to be honest. I do like to keep stacks of every herb around though and continue to grow and add to those stacks just in case.
One underappreciated herb that I have come to love: Rookswort. 10 full minutes without automatic mood loss. It's super-excellent once you get to a higher level and want to go mining for a long time. It also looks beautiful in the herb plots.
I second Munchma Kuchi. Rookswort is very underappreciated. Some other notes:
* Hairball is a speed increase around the same as the candy is/was. Spinach is much faster, though obviously far shorter, and you may not always want that super-high jump. Thus, the choice between them really depends on context.
* Rubeweed appears to be good for spinning gameshow ticket wheels. I spun 19 earlier today, while under the effects of Rubeweed, and got eight music blocks out of it. Not too shabby.