gistfile1.cs

private readonly IContentRepository _contentRepo = ServiceLocator.Current.GetInstance<IContentRepository>();
private readonly ContentAssetHelper _contentAssetHelper = ServiceLocator.Current.GetInstance<ContentAssetHelper>();

Product product = _contentRepo.Get<Product>(new ContentReference(123));
ContentAssetFolder assetFolder = _contentAssetHelper.GetOrCreateAssetFolder(product.ContentLink);
Review newReview = _contentRepo.GetDefault<Review>(assetFolder.ContentLink);
newReview.Rating = 5;
newReview.Text = "This is such a nice product";
newReview.UserDisplayName = "Arve Systad";
newReview.Name = newReview.UserDisplayName + "(" + DateTime.Now.ToShortDateString() + ")";
_contentRepo.Save(newReview, SaveAction.Publish, AccessLevel.NoAccess);