From the official page at Spellhold Studios:
Tales of the Deep Gardens allows players to travel through a new land, deep under Faerun; a realm of mystery, dream, Colourful Essences and riddles. Talk to people who may not even see you, try to find the answers to obscure riddles, fight with Living Colours and gather 10 Colours to leave the Gardens. But remember, Dreamer, that this place is something more than just a chain of caves... It may be a Dream you'll not want to wake up from...
The mod includes some voiced lines, new areas, portraits for characters you meet, music by Lunatic Soul, plenty of items, both edited and brand new graphics, spells and more.
Forums, official page, download page.
And if you're interested, my opinion:
Odd. Decidedly bizarre, but beautiful. The music, the weird lighting, and the sheer weirdness of the mod itself created an entirely unique experience. There's two merchants in the Deep Gardens, and the items they sell are interesting but not overpowered, while still fitting the mod's theme. I found the ending lent the quest a rather bittersweet feeling. My only complaints are a couple typos, too much action text, and the lack of quest "cues"--that is, it is sometimes hard to tell what you should do next, and one of the Colours is annoyingly hard to get. Also, it is possible that you will be unable to complete the quest, as there is a way to ruin the whole thing. All in all, worth playing.
Edit: stupid Ipsy, forgot other links.