There is a long, predominantly Western tradition of speaking of the Spirit as the vinculum caritatis between Father and Son, which - if taken to mean that in the divine life the indiscerptibility of love and knowledge is such that God's generation and procession enfold one another, the Spirit acting as the bond of love between Father and Son, the Son as the bond of knowledge between the Father and Spirit, the Father being the source of both - is a good and even necessary term. But it can also be misleading, in various ways: as Orthodox theologians occasionally worry, it can give the appearance that the Spirit is not irreducibly "personal" as Father and Son.
Correctly understood, however, it does none of this; and it depicts the Spirit as not simply the love of Father and Son, but also everlastingly the differentiation of that love, the third term, the outward, "straying," prodigal second intonation of that love.
Whatever the merit may be of Hart's evaluation of the tradition in general (and stricken as I am by intellectual hero worship, I doubt I could ever flatly disagree with him), Grenz's pneumatology seems to fall far short of Hart's "correctly understood" application of the tradition. Instead, Grenz sums his position up thus:
The personhood of the Spirit arises from the personal character of God as well. The love that binds the Father and the Son is the essence of the one God, for "God is love." God is also personal. Therefore his essential nature - love - is likewise personal. This essence is also the third person of the Trinity, the Holy Spirit, who as the "concretization" of that essential divine character must be person.
I would like here to point out merely the most obvious flaw with this line of reasoning: simply because something is person does not mean that it is a person. The adjective "personal" in general usage - and, it would appear to me, in the above usage - means that object of the adjective involves persons. The love of the Father and the Son is personal because it involves the hypostases Father and Son. The love in my marriage is arguably personal on the same grounds, that it involves two (and hopefully only two) persons. By Grenz's logic, this makes the love in my marriage a person in its own right. Grenz argues, and I certainly agree, that God is personal, but it is because he is "constituted" of persons. I likewise agree that his love is personal, but again it is personal because it is "shared" among persons. None of this demands that the love is itself a person distinct from the persons who share the personal love.
In short, simply calling something personal does not logically demonstrate that it is a person. PC owners everywhere can sleep easy tonight.