for surfing regular web pages - no noticable difference
for web pages with big flash graphics or similar - slightly noticable but not worth the $10 month.
for small downloads - 1-10MB AT FULL SPEED* - definately noticable but not worth $10 a month
for medium downloads - 10-60MB AT FULL SPEED* - definately noticable and POSSIBLY worth the $10 a month
for large downloads - 60MB and up AT FULL SPEED* - definately noticable and worth the $10 a month
* - by full speed I mean downloads that can saturate your connection to the fullest - like downloading a service pack from Microsoft.com or something. From some web sites downloads can be slow but most sites are good.
It all depends how much downloading you are going to do. If you will be surfing only with the ocasional 5MB download, it's not worth it in my opinion. If on the other hand if you will be downloading big files - like Linux CDs - spend the $10.
If you are a gamer and do a lot of gaming online (or plan to) there are some DSL companies that provide a special type of connection that is not so fast but provides faster responce and faser uploads, which should help gaming (that is what I have heard but it would be better to confirm that before ordering)
And lastly, from my persional experience:
At first I was on the slower connection. I had a lot of problems - interuption of sevice, slow speeds for extended period of time etc. Since I upgraded to the faster speed - no problrems.
I hope this helps.