A can-trip that replaces itself is very valuable. Running four copies in a deck essentially means that you are running 56 cards in a 60-card deck, or 36 in a 40-card Limited deck. Having a card replace itself with something you possibly need AND having the ability to fix for mana is especially awesome.
The true power of this card has always lay in the fact that it can help you color-fix in three, four, or five-color decks. Say you were short a black source, or a white source, or any source at all Manamorphose would fix that for you and replace itself. There is nothing worse than being color-screwed and nothing worse than a bad top-deck in a situation where you need action. Manamorphose is almost never a bad top-deck as long as you have something to immediately play after casting it. Multiple copies of Manamorphose are never dead, either, as they can play into each other. Also, something that is sometimes overlooked is that Manamorphose is playable at instant speed, allowing you to keep open mana to bluff opponents.
In 2015, Modern is still a relatively young format. So far, Manamorphose has typically only been seen in Storm decks and decks that can make instants and sorceries be less expensive. Goblin Electromancer, for example, makes Manamorphose only cost a single colored mana, allowing you to more easily play multiple spells and build up mana to be able to ”go off” and cast a whole bunch of spells in a row to make your Storm cards like Grapeshot and Empty the Warrens potentially lethal. For that reason alone, it’s very valuable. It’s also very good in Pauper Storm decks for a similar reason, as it was originally printed as a common; reprinting as an uncommon doesn’t affect its Pauper status.
Overall, Manamorphose is a somewhat under-utilized card in the Modern format currently. We would expect this card to become more popular as the format continues to grow and mature. So you may want to pick up as many copies of this little card as possible. If it only continues to see play in Storm decks, such as Pyromancer Ascension, then it’s still a valuable commodity. It could easily become more valuable in days to come and its value continues to rise steadily on the secondary market.