You will not be able to connect to Xbox Live at all, you will be banned the second your console attempts to connect to Live.
Unless you have some really old console that has not been used in the last two years you will not be able to Jtag it.
You need the 7371 dashboard (or lower) and for the console to have been manufactured before 16th June 2009
Pros:
Play games from any region
Use any size hard drive in the console
Access to homebrew apps such as emulators
Play all your games off the hard drive
+ More ill not go in to
Cons:
No Xbox Live, however the solution is to have 2x Xbox 360's, so its a minor one at best as you then have another console to system link with friends.