It smells a little bit like melted plastic, but its functioning perfectly well right now and I can't see any problems having inspected it. Would you replace immediately with this kind of thing? or just keep an eye on it?
I would definitely order a new one, and if you can't live without your computer in the meantime - just keep an eye on it, and never leave it powered on and unattended.
It might also be a good idea to switch it off and have a visual inspection to look for signs of damage.
Disclaimer: I am not a fireman, electrician or hardware engineer!
Considering that it is not new (the first days/weeks a strange smell is normal) I suggest to looking for a new PSU.
I would suggest to not look at the data of your old PSU but make a list of the component (models) you have mounted on your PC and only THEN decide the characteristics of your new PSU.
If you need help just post your components and we can help choosing the correct one for your needs.
And an additional thing, if you like your PC don't buy OEM PSU but buy good quality ones.
If it is brand new and making the smell, I wouldn't worry about it if it went away after a few days. If it has been in use, I would promptly stop using it and remove the PSU. Just because it seems like it's working, doesn't mean it isn't starting to fail. Be thankful it warned you before it fried the rest of the computer.