I think that some of the info *may* be useful - especially written comments - if they provide you with a basis of information to consider and ask questions about when interviewing.
However, as with any non-verified forum, you have to take these things with a grain of salt.

Here are a number of things to consider:
1. The site is - or has been in the recent past - "broken"/not accepting new reviews for a matter of months or perhaps more than a year.
2. The site gives no indication of the age of comments. Some are many years old at this point. This can be misleading, especially in cases where most comments are about principals/management/coworkers who haven't necessarily been with the firm now in several years.
3. The site moderators edit comments. This means that a certain amount of material is deemed unsuitable for the site - so you may not be seeing the worst there is to say about the firm! This editing may also have the effect of subtly altering the meaning and/or level of emphatic praise or criticism of the original comments.
4. The site moderators tend to flag and delay publishing those reviews that score particularly high or low. Some of these are "investigated" in some respect and eventually posted. Others never are posted, merely because the site moderators deem the score to be suspiciously low or high (I know this from personal experience.) This has the effect of screening out the extremes.
5. People who post reviews might be expected to be those who are either especially critical or complimentary of the firm. Those with "average" types of experiences could be less likely to bother posting, or may have less to say. So the firm may have many reasonably happy employees who don't post reviews, but one very unhappy employee who does, resulting in a very low score.
6. There is nothing to stop firm management from praising themselves, or to "request" that their employees do so (there are some reviews on there that claim that bosses pressured employees to do so.) There's also nothing stopping a disgruntled employee to post multiple negative reviews using multiple identities.
There are many reviews of small firms that have total respondent #s far higher than what might be expected of such small firms. This seems to suggest some double-posting and "plants".
7. Some of the scored categories exhibit the biases of the creator of the surveys. For example, a firm will score lower if it does not have "support staff" and expects architects and/or interns to do things like answer phones and do office work. (This is a value judgement, as many find that type of jack-of-all-trades role in a small firm to be desirable/educational...)

Two more concerns:

1) Sample size. Many of the firms reviewed only have comments from one or two people.

2) Self selection. This and similar sites (e.g., RateMyProfessor.com), by their very nature, tend to attract more people with complaints to lodge than they do less passionate folks. It doesn't mean that the criticisms aren't valid, only that they don't represent a broad spectrum of employees at any one firm.

there are no other tools to see inside a firm before an interview. use inside arch wisely and use it well. the ratings are only as good as they are - so it is worth being honest and contributing info about the places you've worked. they try hard to be accurate and fair.

inside arch is only one part of the due diligence a prospective employee should engage in.