Generally, affidavits have weak evidentiary value as they are considered hearsay under the hearsay rules of evidence.  Even if an affidavit is not opposed by a counter affidavit, it is accepted with caution.  The evidentiary value of an affidavit may go up if an affiant states noting but facts in the affidavit, and later testifies to the same facts in court[i].
 

Some common uses of affidavits are as follows:

• An affidavit may put the court process in motion, ie be included in a complaint,
• An affidavit may be used for impeachment purpose,
• An affidavit may influence decision making,

Some commonly admissible affidavits are as follows:

• An affidavit that is provided for by statute
• An affidavit which states facts which are generally admissible in the case,
• Supplemental affidavits may be admitted if filed to clarify some deposition testimony,
• An affidavit which is not objected to by the opposite party

Some instances were affidavits are not admissible are as follows:

• Affidavit stating facts based only on information, belief, or hearsay,
• When the affidavit is not subject to cross-examination,
• Unsigned affidavits.[ii]

[i] Am Jur 2d Affidavits § 18

[ii] Am Jur 2d Affidavits § 19