Since the arrival of mesh in Second Life, a whole new range of items have become mainstream, from attachable feet and hands, bouncing breasts, whole mesh avatars in large or small or fairy sizes. It's becoming normal for creators in Second Life to have to accommodate a range of attachments which may make their creations more or less wearable for an avatar.
Prim hair and its laggy properties have been replaced with mesh hair which moves with the avatar but may not react in the way that real hair would. However the mantra for any sort of mesh or prim attachment has to be to try the demo before buying. This applies to hair, shoes, clothing, and any body part. If there is a demo it makes sense to find out whether the attachment will work with your avatar before you buy it.
Mesh avatars are worn over your system avatar, and will usually be rigged to work in the same way as your avatar. Most are adjustable using the sliders, although this is usually explained in the documentation accompanying any purchase. Many of the mainstream makers of avatar attachments have in-world assistants or webpages to answer queries.
Mesh attachments for feet or hands are fairly simple to use, but may come with complicated instructions for making them blend with your skin. More and more creators in the skin business are producing appliers for use with avatar enhancements like prettier feet, and they sell them in world. Appliers make blending your skin onto the attachment very easy indeed (just wear the hud and touch the button) but you will need to check that your skin maker makes an applier in the shade of skin you wear - older ones may not be available.
|System feet left, mesh feet and shoes right (Slink)|
Once you are wearing a mesh avatar enhancement, you will need to be aware that system clothing will not extend to the attachment. Thus socks or tights worn on your avatar will stop at the join between base avatar body and mesh attachment. More and more creators will provide appliers for those things too.
Breasts are even more problematical, as clothing will not show over the breasts unless the creator has provided an applier to work with the breasts. Lolas, tangos and other makes are often cross-compatible with each other, and it is certainly something to consider if you wish to be able to wear your breasts with clothing! Often, making breasts blend in with the body of the avatar can be tricky, as SL treats light falling on objects slightly different from light falling on avatars, and so even with great blending you can find that the breasts suddenly look a bit different. I loathe avatar lights, because they interfere with a lot of other things and can give a very odd effect for other avatars. Once again - try the demos!
For myself, I find the vast difference in appearance between system feet and mesh feet makes it worth having to change feet when I change shoes from flat to mid high, and I like a lot of the shoes which work with mesh feet and don't work with system feet. But I haven't found the big breasts worth the effort. I think that if you spend a lot of time naked, and can overcome the lighting problems, it's probably worth it, but otherwise, not.
I do however have both petite and animal mesh avatars which I like to wear from time to time, and I feel sure that as technology moves on, the additional attachments for the avatar will become simpler and more workable. It's still not going to make me want a vagina that has independent thought and talks, however!