You didn't mention which platform or Oracle version - always helpful. I was never able to detect any impact on a performance system. I was skeptical, but ended up running some full backups on a production system during peak production times and was not able to detect any impact. This was Oracle 8i and 9i on Alpha and Solaris. Depending on your system you might experience some slight degredation of performance, no guarantees.
Now, if these are off-line backups, then there will be a noticeable effect ;-)