@Kenny: Using the XSLT functions will in many cases save you some time as they deal with the underlying objects for you. So all in all much easier to work with. You can of course drop down a level and work with the objects themselves. This is pretty common to do if you need to override specific behaviors in the API.

@Dan:

Regarding the URL you can use the XSLT library, but also you can leverage our DI container directly:

var urlService = ObjectFactory.Instance.Resolve<IUrlService>();
urlService.GetUrl(catalog); 

The way you're loading objects is just right. We tend to use extension methods and hook them up on the CatalogContext, so you can go

ProductCatalog catalog = SiteContext.Current.CatalogContext.GetCurrentCatalog();

The extension method essentially encapsulates the code you posted, but saved some duplication and makes it a little simpler to work with. Here's what the extension method looks like:

public static ProductCatalog(this ICatalogContext target)
{
    string catalogIdStirng = Request.QueryString["catalog"];
    int catalogId;
    ProductCatalog catalog = null;

    if (int.TryParse(catalogIdString, out catalogId))
    {
        catalog = ProductCatalog.Get(catalogId);
    }
    
    throw new ConfigurationErrorsException(string.Format("Catalog with id {0} not found", catalogIdString));
}