In terms of plot and acting, The Xindi is one of the best episodes of the Entire Enterprise series. It does not suffer very much from the series' chronic plot recycling tendencies and offers a few scenes with intense evocation by Bakula, Trinnear, and even the usually understated Keating. The use of marines for a rescue op is also nice innovation and is carried out very well. Though on a whole the episode is better than average, it is not without the usual Enterprise problems (see last paragraph)
A lot goes on in 'The Xindi' and several story arcs and subplots are set up for the third season of Enterprise. A mysterious council consisting of several different alien species - at least a few of which are Xindi - appear to be pulling the strings of the recent Xindi attack on earth, but the Enterprise's crew has learned next to nothing after 6 tense weeks in The Delphic Expanse. Finally, a Xindi slave-laborer is located on a mining planet and Archer offers him freedom for the coordinates of his home planet. Can't say much more without a spoiler, but I will say that the ending of The Xindi is, if nothing else, unconventional for the Star Trek franchise.
On the negative side:
* Enterprise's in-house writing team, and the director of this episode once again felt it necessary to ask Jolene Blalock to take her clothes off for another pointless, drama-less and gratuitous nude scene, and came up with an excuse worthy of a teenage boy under the influence of hormones as a plot device.
* There is one major plot element which viewers have seen almost as many times as they've turned on their sets - Captain Archer gets involuntarily detained (sorry - having reviewed a few episodes of Enterprise, I have run out of synonyms for 'kidnapped').
* The Expanse, which has driven an entire crew of Vulcans to insanity and murder, and is built up as a cross between the place that the original crew of The Event Horizon went and Room 1408 at the Dolphin Hotel in New York, after six weeks with the crew of the Enterprise, has only managed to throw some storage containers around in a cargo bay and put Archer in a bad mood?
* Porthos does not appear.
All considered - these minor quibbles should not prevent you from watching one of the series' better key episodes.