I've been using the WAIS-IV exclusively for about nine months, which I think has been long enough to offer a few brief comments on it, as compared to prior versions of the WAIS, as well as tests in general.
Generally speaking, I really like using the WAIS-IV, compared to previous versions. Administration consists of 10 primary subtests, which have been streamlined in terms of discontinuation rules. For example, Block Design is discontinued after two consecutive scores of zero, Matrix Reasoning after three consecutive zeroes, etc. In addition, with each new version of the WAIS, the subtests have gravitated towards ones with easier-to-score items; this version, for example, has relegated Comprehension to "Alternate" staus, and there was much rejoicing. The subtests themselves have been improved in terms of their introduction, as more sample items are provided to give direction to individuals who are unfamiliar with this type of task, or who may have difficulty grasping what they are supposed to do - I believe this has addressed a prior concern of mine; that individuals who already have relatively low intellectual functioning may have needed longer to simply grasp what the tasks entialed, possibly hurting their score.
Conceptually, the test has adopted the four-factor model, rather than the old two-factor model. Now, Processing Speed and Working memory are affored their own spot at the table, and all subtests load separately on the various factors, rather than having overlap. Given that many clinicians reported the four factors anyway, this formalized shift is nice.
Overall, I've been really happy with the latest version of the WAIS-IV. It addressed a few minor concerns I had with the WAIS-III, is streamlined, and relatively quick in terms of administration time.