I only have the Share XL... and I have to try pretty hard (basically jumping up and down with my legs open) to get it to budge. The bulb is one of the largest around.

I think that they get poor reviews because you need really strong Kegels unless you have a really massive bulb on them. I don't have a feeldoe yet, but I can hold the business end of the Pure Wand and stand up and the size is about the same (The Pure is heavier, though). It's a matter of remembering to hold onto it.

It's about muscles.

The bulb on the Share and Share XL are basically the same except for an artistic little ridge. It really depends on if you want that extra texture / girth on the receiving end or not.

I don't know anyone who uses strapless dildos without a harness. But if you've got the muscles, I think the Share XL is your best option.

Don't know about this one, but I have a feeldoe and can keep it in--but I have to remember to squeeze!!!

Feeldoe is more dependent on muscle tone, whereas Share has a larger bulb that can "catch" a little on your internal anatomy and make retention easier. It's likely that all the Shares will be similarly easy or difficult to hold onto. Some people still might not be able to hold it. Note that it's harder to hold onto any double if you're using it to penetrate an anus rather than a vagina.

There are other designs, such as the Happy Valley Tango or Mantric Sysil:Edge that are more sculpted and work very well for some people but not so well for others.