Basil is one of those weird notes that smells completely different in perfumery than it does in real life. An actual basil leaf smells wonderful, a natural juxtaposition of minty brightness and dark leafy herbal green. A basil note in perfume is a dank, dark, oily green note that borders on leathery. It was a huge contributor to woody chypres, giving a dirty masculine undertone to scents from Aramis to Bijan.
Marc Jacobs Splash Basil is neither of these. It's actually more of a sweet citrus splash cologne with green undertones. At first, I smell grapefruit mixed with mint, while a candy-sweet mandarin plays underneath. The dark basil note is technically there, but it's so weak that it's more of a leafy green darkness under the sweet citrus than the star of the show. After about an hour, it dies down to a pleasant green soapy smell.
I can't mark down Splash Basil for being very weak - it says it's a splash right in the name - but I'd be remiss to not mention that you'd have to wear an awful lot of this to make an impression. In terms of smell, it reminds me of Jo Malone's pleasant but equally unremarkable Lime Basil & Mandarin, though I'd probably pick the Jo Malone over this because it's stronger and lets the basil get darker and more interesting than Marc Jacobs. Meh.