Just my opinion, not that it ever tends to be worth much, but I think Invasion was an excellent set to bring out. Many of the cards are quiet playable, unless you go to PTQ's and other DCI style tournaments. But then theres never really alot of playable stuff from one set or those anyways,(urza's block excluded).
I think they took out the duldrum boring feeling or watching masques block beat on itself with rebel after rebel after rebel and the occasional rising waters. With Invasion coming into the fold you have many more possibilities to play with and things that are much more interesting to watch and play. Like the "fires" decks running dragon legends or two-headed's. Thats something that hasn't been seen in ages.
Secondly, if your playing a mono-color deck and you want to splash in some gold cards, then your no longer playing a mono-color deck and yes, you'll have to re-built your mana flow unless your playing green which can have things splashed to it quite easily. You don't play ALL the gold cards in one deck unless you intend to have alot of lands that can produce multiple colors of mana, like 2 of each of the original dual lands. Most of the good gold cards in the set are 2 color, with the exception of the dragon legends, and many of those only require 1 mana of a perticular color, so splashing isn't all that difficult. Things like City of Brass, the pain lands, and the new psuedo-duals all assist in this for people who aren't playing green decks. If your using green you've got BoP, Utopia Tree, Quirion Elves, and Harrow for starters, and thats just the T2 legal stuff.
Another pair of cards I'll add to Yellowjackets "complaint"
list even though they aren't used in tournies much, are still fun.
Armadillo Cloak: Why would I want to give a creature +2/+2 and have it trample, and then gain life for it dealing damage? That must be horrible, and only for 3 mana, sounds like another card for the incinerator.
Opt: What?! One blue mana to look at the top card of my library and either get it out of the way for the one below it or draw it? hmm, sounds pretty lousy.
However I will agree that there are several rares that are quite good and fun to play with in non-tourny environments. I've got a really cool Spreading Plague deck that I've been toying with.