I downloaded the new launcher and downloaded 13w16a.
After waiting 2 hours for it to download when i launched it it would not work - it DID load the Mojang Splash screen but stayed on that - I took a look at the launcher to find it was a log and i saw (what looks to me like) an error:

Client> java.lang.StringIndexOutOfBoundsException: String index out of range: -1
Client> at java.lang.String.substring(String.java:1937)
Client> at asm.a(SourceFile:1469)
Client> at asm.a(SourceFile:1461)
Client> at asm.k(SourceFile:1452)
Client> at asm.K(SourceFile:300)
Client> at asm.c(SourceFile:503)
Client> at net.minecraft.client.main.Main.main(SourceFile:76)
Client> b@320f6398

I will provide as much info as i can to get this fixed

EDIT:
Full log (from when i clicked "Play")
http://pastebin.com/NsH713Wq