If memory serves me correctly, I installed VB4 in Jan 2010, and added the same SEO features I am still using in XF. From switching VB3.x, where my users had a good return rate, unique vs. returning, similar to what I get now from XF, I considered reviewing what went wrong when I upgraded to VB4.
Considering Vbulletin make claims to the public their software is so great, and against Xenforo themselves... then why not let user statistics do the talking instead!!!
Feb 2010 - 130% difference
Apr 2010 - 95% difference
Jun 2010 - 66% difference
Aug 2010 - 70% difference
Oct 2010 - 54% difference
As you can see, my users actually decreased in returning to the site overall, as my uniques increased over the year, showing basically, statistically, that users less favoured returning to my site whilst using VB4 software, and progressively got worse and worse over the course of the year.
Changed to XF a week or two after it was released... Nov 2010 approximate area from memory... and suddenly, 2011 using XF software...
Jan 2011 - 125% difference
Feb 2011 - 117% difference
Mar 2011 - 125% difference
So starting to measure again in Jan 2011, after a month and a bit to stabilize, add Christmas into it, and suddenly my new visitors vs. returning visitors after changing to XF, have once again risen to near the same levels as what it was when using VB3.x software.
I didn't include Apr, as the month is not complete. No doubt, this will only continue to slowly improve as uniques go up vs. returning traffic, using the better software choice for delivery to my users.
IMHO, this really reinforces those who chose to remain on 3.x or change to XF vs. using VB4.x software... have made all the right decisions to shift. You can shoot crap between admins all day long, but user statistics do the talking the most.