Recently diagnosed with psoriatic arthritis & prescribed enbrel (etanercept). How effective, in the long term, are these injections? Any downsides? Thank you!

Extremely effective. Tnf agents are extremely effective in the treatment of psoriatic arthritis. It is surprising that the side effects were not reviewed with you prior to starting the med. The major risk is infection with other possible side effects being reduced blood counts, liver inflammation, neurologic disorders and a possible increased risk of lymphoma.
Enbrel (etanercept) Because Enbrel (etanercept) works on your immune system, it may lower your ability to fight infections and may raise other safety concerns. If you have any sign of an infection including a fever, cough, or flu-like symptoms or have any open sores on your body, call your doctor.Enbrel (etanercept) was shown to be effective in about 50% of psoriatic arthritis patients who used it at 6 months. Hope this helped :].