I used to use Aperture, and I loved it. I still do in many ways, and would love for Apple to get it together and enhance it to at least catch up with Lightroom. I particularly love the ability to stack bricks of the same tool, especially curves. I like the way it organises files too. However, currently I find Lightroom 5 does the image adjustment thing better for most parameters (but not all – for example, I tend to do better with white balance in Aperture. Of course, milage might vary).
I must add though, I always use Capture One to import, make base adjustments and convert to TIFFs. I like it for this (& they have a 30% discount for the Pro version as of writing this).
Anyway, my first Lightroom experience is with version 5,* so I can’t really say if upgrading from version 4 is worth it for anyone. Having said that, I find I use the new radial filter a lot (that and the gradient filter are two of the reasons I switched). The radial filter’s really useful, especially since you can flip the effect (‘invert mask’ – i.e. inside the circle or outside the circle). I use it to burn and dodge in a lot of situations. Subtle application** can lift something out of the mix at times (or push it under), with the spill preventing it from looking pasted in, like the masking brush can do.
* apart from once-upon-a-time trialling version 1, which was a pig and scared me away until recently
** i.e. not like in the top right example in the link above