Are there any problems using temporary tables in a multi-user application?
As per my experience, Yes it creates problem when it is not carefully planned, specially when using connection pools
temp tables are kept separate for each user.
Yes It is ,but when you say it is separate for each user this means it is separate for each connection and connection pooling mess-up every thing.
In a multi user scenario, A connection with with pooling is shared among many user of your application those are not database user but application user, therefore in reality a temporary table is not for one user and you are likely to get unexpected results.
Gravitation cannot be held responsible for people falling in love ~ Albert Einstein