Helmi;
Related fields can be problematic in bigger sites because of the way EE handles them. They can bloat the database pretty quickly. In our case, we had a weblog called Companies and another called Products. We had about 80,000 entries in the Product weblog and a relational field to pull in Company data. The exp_relationships table got *huge* from that since it caches both related data and reverse_related data in the table.
It’s pretty easy to work around this limitation by just using sql queries instead of relationship fields.
We host a few online publications at EngineHosting.com using EE and aside from that snafu with relationship fields (and they work fine for more modest purposes) we’ve been very pleased with the results. We do 1-1.5 million page views/month on our busiest site, if that helps you judge. Now that site only has about 4,000 entries, though.