The ability for us as developers/testers not only test our own applications but also other peoples' apps too! We can even use it in conjunction with Selenium IDE or any browser we want - so I don't have much dislikes here at all (except that sometimes when there are many tests running they tend slow down my computer).
It is quite expensive compared to some others out ther ebut if this suits what yoyu need then go ahead otherwise try something else first because their support isn’t always great either which makes me hesitant about using them again after being turned off by one person who couldn;r help very well once i had explained everything properly twice over 2 days ago now :(.