I've just upgraded an Umbraco 6.1.6 (MVC) site from uComponents v5.5 to 6.0 - I did this using a package but I also used Nuget too (as I had a class library that referenced uComponents). The upgrade went fine. I also republished the entire site, too.

However, I noticed that everywhere I'd used the MulitUrl picker that I was getting either no links being shown or null reference exceptions. I was using the MultiUrlPickerState.Deserialize method to deserialise the links into a collection of UrlPickerState objects. Example code was something like:

var links = MultiUrlPickerState.Deserialize(item.GetPropertyValue<string>("links"));
@if (links != null && links.Items.Any())
{
    <div class="direction-links">
        @foreach (var link in links.Items)
        {
            string title = link.Title; // EXCEPTION!
            if (link.Mode == uComponents.DataTypes.UrlPicker.UrlPickerMode.URL)
            {
                title += " website";
            }
            else if (link.Mode == uComponents.DataTypes.UrlPicker.UrlPickerMode.Media)
            {
                title += " (file)";
            }
            
            if (link.NewWindow)
            {
                title += " [opens in new window]";
            }
            
            <a href="@link.Url" @Umbraco.If(link.NewWindow, "target=\"_blank\"") title="@title"><span>@link.Title</span> <i class="icon-caret-right"></i></a>
        }
        
    </div>
}
v>

string title = link.Title;

var links = item.GetPropertyValue<MultiUrlPickerState>("links");