There has already been a lot said on this subject. The positives have tended to focus on David Tenant and his portrayal of the Doctor, while the negatives have tended to focus on the writing/story. In my opinion both are true.
It's a difficult one this, and I can see the reason for RT (the writer)'s conflict.
On the one hand, the passing of all previous doctors (while dramatic) has almost simply been a fact of the last episode of a seiries and you could argue that the transition from the 10th to the 11th Doctor should be no different. On the other hand (regardless of what the detractors may say) we have in David Tennant, without doubt, a superb actor who has captured the public's imagination in this role, has played it for loger than anyone in recent memory and as a result someone who is held in very high public affection. In these circumstances, in my opinion, it is only right that this is in some way acknowledged. So it's a bit of a ballancing act, acknowleding what has gone before and turning out another Doctor Who tale, but at the same time not ignoring the current circumstances. My own conclusion is that the ballance is not wholely successful with the the writing focussing too much on the fact that it is David Tennant's last outing as the Doctor to the detriment of aspects of the story and the overall cohesion of the episodes.
First and formost, let nothing detract from the quality of the acting of the key players (cactus people asside) here. Sims as the Master, Cribbins and Tennant as the Doctor. All Superb. All accusations of over indulgence asside, you can't really argue with what the actors do with the material they are given. Tenant especially. You may find yourself questioning the the direction the storey goes (especially at the end), but there is no denying that David Tennant superbly conveys every emotion the doctor is feeling.
The other superb element is the Wilf - Doctor story arc, culminating in the fact that it is Wilf, in a very quiet an unassuming way, who is the key player in the Doctor's death and regeneration.
The surrounding storey and events of the episodes. The general premise is a perfectly valid and potentially effective one. Everyone watching knows that the Doctor is going to die/regenerate, so have a big event that creates a loud distraction that everyone expects will bring about the expected result, then when the dust has settled, quietly reveal that this is not the case at all and the 'event' unfurls in a wholely unexpected manner. As I have mentioned, the 'quiet' bit works wonderfully, it's just the 'noisy distraction' that doesn't and given the fact that this is what takes up most of the double episode running time, that's not insignificant. Despite the grand nature of the events, the return of the timelords/galifray thing just does not feel significant enough. They've necver been a feature before appart from the odd mention and so they do not have the feeling of being one of the Doctor's significant enemies. The master is a different kettle of fish, but then it didn't end up being between them really and as a result it just didn't feel weighty enough. If you think back to the end of Series 4 on the other hand and the whole return of Davros etc, THAT felt significant and definately a main event that could have incorporated the 'quiet death' arc very effectively.
The drawn out post radiation / pre regeneration section. Much talked about, much malligned. As I have mentioned previously, Tennant's performance remains faultless throughout, but it is too much. Too long, too indulged. A couple of minutes to say a few words after coming out of the booth would have been enough.
RT (the writer) clearly loves the Doctor, but more specifically he clearly loves David Tennant as the Doctor and as such, after 4 years felt the need to give him a decent send-off. The trouble with love is that it effects your judgement and that is exactly what has happened here. The focus is on the death of the Doctor and the fact that it is David Tennant's last outing to the detriment of the other elements of the story. As a result we have a very strong and well written Doctor/Wilf arc, an over indulgent, over written and over long lead up to the actual regeneration and everything else feels underwritten / like not enough time was spent on it. Having said all that, the perfomances and the key Wilf/Doctor arc still make this a rewarding and enjoyable experience, just not as good as it could have been.