tl;dr if you're a kenshin purist, feel free to not enjoy Seisouhen.
Seisouhen does deviate from Nobuhiro Watsuki's wish for Kenshin's fate at the end of the series; he states this
in an interview. So now you know Seisouhen was largely another director's vision.
Make of that what you will, there are plenty of people (including me) who enjoyed Seisouhen for what it was; it is a poignantly animated piece, and apart from some irritatingly unexplained character decisions (which I won't spoil), it stands on its own quite well.
The OVA starts off with a one-hour summary of the events in the Enishi arc, which would probably bore you a bit if you've already read it in the manga. I still liked it, as the tone is much more serious this time. I found the randomly-lighthearted tone of the anime/manga somewhat irritating, as it has a tendency to jump from a completely serious mood to a comedic one without warning. Seisouhen is completely serious; no random comedy to jump you here.
As an added bonus, it also does away with all the shounen bullcrap present in the manga, replacing the Kenshin-Enishi-superhuman-faceoff-battle in the manga with a shorter, more palateable animated fight that fits within a more reasonable timeframe as well. If you are a big fan of shounen fights, you will most likely not enjoy this.
The last complaint I hear quite often (or used to hear; I don't read RK threads as often as I do anymore) is how unjustified the ending is. If in the course of the anime you've gotten attached to Kenshin as a character and want to see him come out alright in the end (a la "good ending"), let's just say that watching Seisouhen will hurt a little inside.
As heart-wrenching as the Seisouhen OVA is, I feel it gives a poignant, alternative perspective to the man that is Kenshin. In the anime/manga, he manages to atone for his sins from the Tokugawa era and lives a peaceful life after the end of the Enishi arc. In Seisouhen, he does not, realising instead that having once picked up the sword, he can never truly separate himself from it. And he meets a fate befitting one who comes to such a realisation.
But don't let my interpretation colour it for you, watch it yourself and decide what you want to make of it.