How would you rate this python code? (Django AJAX)
In my opinion it's little bit hard to read. You have to split it up into POST/GET methods.
Then you have to clean up code in POST/GET methods.
Something like this, for example.

@login_required
def bookmark_save(request):
    form = BookmarkSaveForm()
    if request.method == 'POST':
        form = bookmark_POST(request):
        if isinstance(form, HttpResponse):
            return form
    elif 'url' in request.GET:
        form = bookmark_GET(request)

    variables = RequestContext(request, {'form': form})

    if 'ajax' in request.GET:
        template_name = 'bookmark_save_form.html'
    else:
        temaplte_name = 'bookmark_save.html'
    return render_to_response(temaplte_name, variables)


And functions for POST/GET actions

def bookmark_POST(request):
    form = BookmarkSaveForm(request.POST)
    if form.is_valid():
        bookmark = _bookmark_save(request, form)
        if 'ajax' in request.GET:
            variables = RequestContext(request, {
                'bookmarks': [bookmark],
                'show_edit': True,
                'show_tags': True
            })
            return render_to_response(
                'bookmark_list.html', variables
            )
        else:
            return HttpResponseRedirect(
                '/user/%s/' % request.user.username
            )
    else:
        if 'ajax' in request.GET:
            return HttpResponse(u'failure')
    return form

def bookmark_GET(request):
    url = request.GET['url']
    try:
        link = Link.objects.get(url=url)
        bookmark = Bookmark.objects.get(
            link=link,
            user=request.user
        )
        title = bookmark.title
        tags = ' '.join(
            bookmark.tag_set.all().\
            values_list('name', flat=True)
        )
    except (Link.DoesNotExist, Bookmark.DoesNotExist):
        title = ''
        tags = ''
    form = BookmarkSaveForm({
        'url': url,
        'title': title,
        'tags': tags
    })
    return form