Since more and more people seem to consider Undiscovered Paradise (UP) seriously for their decks I thought this was the right time to start this discussion (half a year ago there was little resonance, but now that even 2cc runs it sometimes...).

First the obvious:
The good: It gives any mana you like.
The bad: It returns to your hand at the beginning of your next upkeep after using, which is especially unpleasant when facing a Black Vise.
The ugly: Apart from combo decks or ancient 5 colour green you can use only one, since you NEVER want to have two or more on the board.

Now the less obvious:
In Magic, every advantage is sometimes turned into a drawback and vice versa. Lets see what can happen:
Good->Bad: apart from non-basic hosers: umm... let's see... if your opponent has a Fellwar Stone your rainbow-land might cause a disaster - ok, I think this is negligible.
Bad->Good: it "combos" with Masticore, LoA, Ivory Tower, Armageddon, Stormbind (well, ok), opposing Racks and opposing BtoB (and several cards I might have forgotten).

Now compare it to other multi-lands (I only list lands which are seriously considered in 5cc, as opposed to, say, Rainbow Vale):
COB: is definitely better, but you can play only 4 of them - and sometimes it is nice to cast a spell without getting hurt ;)
Lairs: IMO they are much worse. I already made this point in a thread below as a response to a post of JP, but I repeat it briefly: Lairs are better in the long run (apart from "combo" situations mentioned above), but Undiscovered Paradise, if played correctly, is better in the early game (because you do not return a land immediately, so you can e.g. lay your crucial fourth mana one turn earlier than with a Lair; and quite often the potential of untapped UUUU prevents something that would have happened with UUU, without the need for tapping), at least equal to a Lair the turn after it is put into play (if and only if you used it is it equal, otherwise it is still potentially better since you might draw another land) and becomes worse for the first time when you have used it twice and have drawn additional land meanwhile. So by using Lairs you trade a potential late-game advantage for a potential early-game disadvantage - this is usually not a good idea in control decks.
Moreover, Lairs give "only" 3 mana (U and B are there for sure, but then? You have to choose between, say, being able to cast Balance, Regrowth or REB).
Duals: Well... they give only 2 mana

There has been some complaints about the coonsistency of 5cc/Keeper, which led some respected Keeper players to the conclusion/hope, Impulse could be the solution. I think not. Impulses decrease either the number of business spells or the brokenness-factor, both of which are unpleasant features (or shall I say bugs?).

So my suggestion would be: If you want to increase the consistency either play (OSE || BBS) or use a less fragile mana base. And with 28 mana sources the problem is in many situations not the amount of mana, but the proper colour - how often do you sit with a Regrowth w/o G, a Balance w/o W or a REB w/o R (especially, when BBS focuses his Wastes on your COBs and Vulcanics)? Probably not THAT often, but every now and then it happens, and nothing is more humiliating than dying to a horde of 2/1s with a Balance in your hand but no W...

Now, I know that 1 Undiscovered Paradise will not change your deck drastically, but in the long run (in terms of matches) you will notice that you have won one game or the other, simply because you had the proper mana for your broken spell, and it was provided by UP. If you notice that you lost a game due to UP, please let me know it, since I would like to collect these rare events ;)

How to play it correctly?
Well, first of all play just one in your deck. And second, it should be the last land that you play, unless you need some specific colour very urgently (read: unless UP saves your ass). So if it is in your opening hand (the "worst case" so to speak), you will typically play it on turn 3 or 4. Finally, remember that you can tap it for mana burn for fun with LoA or Masticore.

Why does nobody else play UP in Keeper? Doesn't that prove it is a bad card?
Well, former Keeper decks contained mana hungry cards like Tome or Fireball/Torch. Especially the Tome wanted 4 mana every turn (no matter which colour) and one primary strategy against a variety of decks was to cast it and win by sheer card advantage. In these decks, UP would have been a bad idea. This is probably the reason, why many people still believe it is bad today (after all, why should a "bad" land turn into a "good" one).
However, in state-of-the-art Keeper UP really shines (ok, I am probably exaggerating, since after all it "gives only mana", but hey, this is a eulogy, remember?).

So, if you have a Keeper deck give it a try and LMK what you think about it. But beware! I can get quite emotional when it comes to defend UP - much like Matt+Sylvan, JP+Duress or Rakso+BDominia